I’m a sucker for multiple Dimitri ships, so I’m not exclusive Dimilix, but I do ship it among others. I’m assuming you’re okay with spoilers? Because I will mention what happens to Felix in all routes.
I do agree that their A-Support leaves too much unresolved (what I would do for an A+ support), but I feel otherwise there’s a lot of good content to justify shipping it.
First, I disagree vehemently that Felix lost his faith in Dimitri/detests him. His reaction to Dimitri is so extreme because he’s so attached to his old version of Dimitri and struggles to accept how trauma affected his old friend. Felix being traumatized by the same event only makes things more difficult and complicated, as does Rodrigue’s rocky relationship with Felix vs his good one with Dimitri, Glenn dying to protect Dimitri and leaving Felix behind, and Dimitri never putting up the fight Felix is looking for in their early supports.
In their A-Support, Felix has had 5 years to mature and had to temporarily consider Dimitri was dead for good via Cornelia’s execution. He shows a willingness to accept that Dimitri’s trauma has permanently affected him, but that doesn’t mean the other side of Dimitri that Felix loved so much is gone or that Dimitri will continue using such horrible coping mechanisms and is striving to do better vs the past where he just ignored it.
A lot of what Felix says about Dimitri in AM and outside of it both in White Clouds and post-TS monastery dialogue is often Felix begging Byleth to help Dimitri in his usual crass way. If Felix didn’t care about Dimitri, he wouldn’t constantly ask Byleth to help Dimitri. He’s also waiting in the church and watching over Dimitri immediately after the TS and only leaves once he’s handed protecting Dimitri off to someone else (Byleth) as Felix knows he can’t do it. I mean, even their dining dialogue showcases Felix looking out for Dimitri, insisting Dimitri eats his meat because he’s worried Dimitri will faint, and later being concerned with how Dimitri’s eating (being the ONLY character who notices Dimitri’s got a weird relationship with food).
Likewise, at the end of AM, we do see Felix explicitly supporting Dimitri. In the monastery dialogue, he tells Byleth as much, but tells Byleth to not dare tell Dimitri he said that - which is both Felix being a tsundere, but also showing he’s not totally comfortable with the notion (at least not enough to admit it in public/to Dimitri’s face). However, their unique dinning dialogue has Dimitri teasing Felix and Felix being more typical anime-tsundere and not vicious like he has been. This means Dimitri feels comfortable enough to play around with and tease Felix rather than walk around egg shells around him like he had been. And Felix doesn’t snap and make it awkward, but plays along in his role. Once you reach A-Support, Felix also realizes he and Dimitri work so “well together it’s eerie,” showing Felix settling into his role as the king’s shield. There’s also the extra heartbreak of Felix saying “I’ll cut through” post-TS (esp outside of AM), which was one of Dimitri’s phrases in White Clouds.
Then you get to the battle before you fight Edelgard. Felix tells Dimitri to “go be the boar you are, and don’t look back.” I can’t express the importance of this. First, he’s not saying this viciously in an attempt to hurt Dimitri or provoke a reaction like before. He’s saying this as encouragement. He’s worried Dimitri will falter and not kill Edelgard because Dimitri’s too soft-hearted. So not only is Felix now openly supporting Dimitri, but he’s acknowledging and well aware that the Dimitri he remembers is still there (or he wouldn’t be worried that Dimitri couldn’t finish the job) but also acknowledging and accepting the darker parts of Dimitri by calling him “Boar,” but not using it as cudgel, but as support. “Boar” used to be Felix’s go-to way to dehumanize Dimitri, now he’s reclaiming the word’s meaning between them to be something far less hurtful and dehumanizing and more encouragement. It also means that Felix feels comfortable enough with Dimitri to openly use that word even when he’s using it in a different way rather than awkwardly letting it linger between them.
Then, ofc, there’s their paired ending, where Felix is more upset when Dimitri eventually dies than Dimitri’s own wife which is . . . interpret as you will.
However, a lot of why I ship Dimilix doesn’t have anything to do with that. Honestly, I probably wouldn’t analyze little interactions with them and just stayed happily shipping Dimitri with just Byleth, Dedue, and Marianne (fan art and fanfic have convinced me that Claude and Dimitri would also be fun). So there’s no shortage of people to ship Dimitri with, and Felix can happily pair up with Annette (I don’t really get Sylvain/Felix tbh, not anti it, just don’t get it). BUT - there’s outside AM Felix.
I love Three Houses, but I don’t think it’s perfect, and one thing I don’t like is how the recruitment system works. It often feels like characters have flimsy reasons to join certain houses. It’s also strange how some characters don’t change at all. For some, it makes sense because they have such lose connections to their house leader and/or country (take Linhardt or Ignatz for example). But for others, they really feel like they need a better excuse to join a house other than “Byleth is there,” or at least have the writers show how picking one side over another might shift how they develop. Except they did that - but only with one character: Felix.
Have you seen Felix’s AM endings vs his endings in other routes? In AM, Felix ends up either happily married or paired up with Sylvain or Dimitri and takes over his father’s position. He’s dedicated to restoring the land and fully believes in Dimitri’s rule. He’s well-adjusted and a productive member of society. Felix outside of AM though . . .
All of Felix’s endings are drastically different outside of AM/Blue Lions. He’s a bloodthirsty, death-seeking mercenary. There’s some cases where the woman he pairs up with can eventually sway him from his path, but all of the endings are more somber. It’s implied he died while seeking death as a mercenary in his ending with Sylvain. In his ending with Annette, he never smiles again except when he sees her. In his ending with Dorothea, he never rejoins society. And on and on. A few aren’t as depressing (like Ingrid joins him as a mercenary until they eventually have kids, and he becomes and actual clown pairing with Leonie which is a bit wtf but true). This is because outside of AM, Felix never has anything to fight for.
Throughout all of White Clouds, Felix rails against chivalry and knighthood, but he doesn’t have anything to believe in, just something to hate. In AM, he believes in Dimitri’s rule. He finds something to support. Outside of it, he just stays hating something and never finds anything to live for unless one of the few ladies who can convinces him not to go and try to get himself killed.
In CF, you get a relationship up arrow if you agree with Felix that he’s become a monster. He can potentially kill his father and all of his friends in that route. It turns him into a stoic, coldblooded killing mercenary who shuts off his emotions in order to justify what he’s doing, while never finding a new path to tread. He goes through a negative character development arc and is self-aware.
In SS and VW, he also undergoes negative character development, but shows less self-awareness about it. If you pay attention, you’ll notice that Felix’s monastery dialogue lines grow progressively more violent, bloodthirsty, and vengeful after Dimitri dies up till the end. He essentially becomes what he was critiquing Dimitri for, someone who becomes a bloodthirsty beast seeing vengeance for the dead. He beats himself up over Dimitri’s death because he doesn’t know if things would’ve been different if he stayed by Dimitri’s side, so he tries to deal with his guilt by taking it out on the people who caused Dimitri so much pain and suffering (TWSITD and the Empire). Once again, Felix finds nothing to live for and believe in.
Some people argue that CF Felix is him shaking the bonds of his heritage and cutting himself a new path - literally. But in order for that to work, Felix would have to find a new path. And he just . . . doesn’t. He becomes a mercenary seeking death. I’m not even over exaggerating because omg I ship it so hard (I’m not married to this ship tbh, I like multiple ships for both characters), it’s right there in the end cards that he fought like a man seeking death. You can’t “cut a new path” if there is no new path.
Ultimately, Felix can’t be happy in a world with Dimitri. Dimitri is the only thing Felix ever learns to fight for, not just fight against something. And in AM he does explicitly endorse and support Dimitri and his rule. While I do wish they had an A+ support to really iron things out, I do think it’s fine where things are.
There’s also, ofc, the drama and angst of it all. At the core of it, you’ve got two people who very much care about each other, but trauma that effected them so vastly differently and the complexities of their trauma stemming from the same event and having such wildly different experiences with it has made them unable to communicate anymore. Felix lashing out at everyone and Dimitri’s holding it all in is a toxic combination, one ripe for fanfic type angst. The fact that, if these two never do make up with each other, it leads to one of their deaths and the other forever miserable and unable to ever make amends adds an extra layer of tragedy and urgency to their relationship. Felix can literally kill Dimitri, or he can end up one of Dimitri’s most loyal and dedicated vassals. There’s just so much to play around with, imagine, write meta or fan fic about, etc . . . I also think that the whole “Boar” thing not getting fully resolved in supports prompt people to ship it more, because they you want to seek meta analysis and go into every line or create fan work to fill in the gaps. Then add the extra layer that their loyalty is fated (i.e. Fraldarius always protects the Blaiddyd) adding yet another layer to explore, not to mention also looking into Rodrigue’s undying loyalty to Lambert and the possibility of multi-generational shipping and yeah. This couple is just a shippers paradise. There’s so much angst, so much to explore, so much you can fill in, so much you can analyze, so much you can add to it, so many cannon opportunities to explore both happy and tragic results, etc . . . It, of course, doesn’t hurt either that these are two incredibly good looking men either.
Ultimately, Dimilix to me is a great ship for a story. It’s one I love reading fan fiction about. If Dimitri and Felix were real people, would I want them to get together? Not really. It’s pretty toxic, and I think they’d have to work through a lot before I think they’d be honestly wholly good for each other. Dimitri’s being prone to depression up to being suicidal kinda needs someone more emotionally supportive if they were in the real world, but this isn’t the real world. This is fiction, and Dimilix provides endless opportunities to explore a really interested relationship that gives you enough to want to ship it, but leaves out enough that there’s lots left to explore in meta and fan work. Like, in the real world, I’d probably want Dimitri to hook up with Dedue or Marianne, and sometimes I’m really in the mood for an overly emotional sweet Dimidue fic (there is a tragic lack of Dimitri/Marianne fic) or seeing soft fan art or reading pure fluff between two wholesome people who love and support each other so throughly - but other times I want the angst, the drama, the sexy tension, the deep psychological scars getting in the way, etc . . .
If you’re really curious, I’d say to read a fanfic called “Music for A While” by ladylapislazuli https://archiveofourown.org/works/21600880 It’s not finished yet, but it’s soooo good, and will give some insight into what I’m talking about a bit - honestly all her stuff is good, and she’s a multi shipper to so :)
Wow this got longer than I meant it to be lol.