Here's a less symbolic version:
And here's the explicit version that's also symbolically relevant instead of fanservicey, but tmk Tumblr's not healthy about these things, so: https://shorturl.at/2LVwI.
Reno's love runs deep and thick, like blood, and Iharu finds home in it.
The drawing as a whole is a sequel/more Reno-themed side to this one (Btw: I've changed it to have veins to go along with the lightning, for the reason explained in that post):
Iharu's fire is more out there...
...while Reno's is more intimate, obsessive, and ardent.
Where Iharu's fire opens up by emboldening and challenging...
...Reno's fire opens up by attention, intimacy of the connection (which might disarm, as he's reserved and doesn't connect easily; but when he does, he's all in. See his relationship to Kafka), and honest, deep passion in everything he does. And so...
• the theme comes from the fact that Reno=aries=mars/blood. Hence bloody fire: Reno is a fire sign, specifically, an aries which is associated with blood. It represents passion that's not loud and out there, but quiet and flowing in one's veins, deep inside. Profound, visceral, even obsessive (which Reno very much is in canon). It's intimate — hence the strong sexual theme (aries itself is the sign most strongly associated with sex, tmk). Sex is the most physically and emotionally intimate, vulnerable activity + the most passionate one.
• same for the color palette:
hot and dark (intimate): violet (Reno's eye color) + red (blood) = combined into this hot pink (kind of the color of Iharu's hair, too, probably inspired by: spring (Iharu's name)=cherry blossom=pink).
also a bit of orange (fire, since they're both fire signs) and golden (Iharu's eye color)...
...But the first two dominate, similar to how the previous drawing is dominated by the last two: more Reno's fire (intimate, quiet) VS Iharu's fire (out there, thunderous) (even though the previous drawing represents how Iharu IS, also, Reno's fire: the intense, wild, uncontained (never giving up) passion that will thunder (quietly) through anything).
• the golden flames on Iharu's body from the previous drawing have turned into more sensual, quieter, and in that violetred color — to represent how the kind of intimate love Reno seems the type to give consumes and embeds itself onto one's mind, how Reno's intimacy turns Iharu inward, giving him safe space to settle into the present moment instead of the constant need for rush and competition.
Reno isn't very competitive nor does he judge Iharu for being "weaker" (even though Iharu ends up being stronger via his flash adapter quirk); he's also patient and composed; which translates to that focused, grounded intimacy that calms and soothes instead of challenging (like Iharu's).
• in the more symbolic version, Reno is drawing blood from Iharu's heart, referencing:
Reno's yearning for Iharu/Iharu's fire (he wants Iharu's heart, his essence (leo is associated with the heart)), discussed and represented with the previous drawing.
Reno's obsessive passion/how intense his love seems to be (he doesn't just fully impose, but he cares deeply and becomes insanely protective ...and then he can become imposing, out of that deep care, as we see in his interactions with Kafka in canon).
the fact that Reno loves music (as per his character page) and Iharu, as a leo, is the heart (the very body part that can be associated with music (since it beats in a rhythm)). *
In the less symbolic version, Reno's hand is pressed flat over Iharu's heart, in that intimate embrace that soothes and reassures.
• the reason Reno is holding Iharu's member (non-explicit in this version) isn't cause it's sexy but:
one, to represent that care and love. He's caring, considerate, wants to make sure his boyfriend is feeling the best he can.
also the passion of wanting to touch him like that, obviously. Only a realistic element, if anything.
but two, that part of the body is the closest to holding the heart: tender, throbbing with the heartbeat. **
• their body language represents both their personalities [in the context of sexual intimacy] and a few other things. Reno's pose is embracing, maybe even borderline possessive (reflecting his canonical obsessive passion (not just as in "protective of those he loves, as at least partly a result of his trauma" but also as in his general passion even before meeting Kafka and Iharu, his "I'll never give up"/"I'll die before I get it" (ch 1))). Iharu's pose is still the confident/empowered from the prev drawing. I debated changing it to something more "melting", more intimate — but then:
one, the open pose = his unapologetic honesty. Which is an extremely important, emotionally mature aspect of his character (btw: see: "Iharu's emotional intelligence"), referenced in the previous drawing by his expression, nudity, and the thunder. He won't hide his vulnerability (unlike Reno might), he'll cry openly, he'll be loud, he'll show you how much he loves it, he'll kick you if he hates it (so will Reno, but Reno is more low-key, less confrontational about it unless he's comfortable (see e.g. how he physically berates Kafka for transforming in public)). *** And in conjunction with his empowered body language, it represents his vulnerability's, honesty's empowering effect (both to himself and to others (ch 63)): acceptance of how it is, and taking action towards our goals. The power of vulnerability — that's Iharu.
two, his current pose references what he's likely to feel from this kind of affection and desire: empowerment (well, many would; especially those more insecure about their worth, etc, which Iharu seems to be). The openness, beside referencing Iharu's openness and honesty, also references the open, vulnerable headspace of bottoming (well, sex in general. But especially the more sexually and emotionally intense one of bottoming) + how that intimate passion that Reno embodies can open one up, like giving up his heart into Reno's protective, devoted hands (I originally wanted Reno's grip on Iharu's heart to be more possessive, but decided to make it somewhere between that and caring. Not sure how it comes off).
it also references the previous image but that's less important.
So I went more the psychology and character study route than the all intimate/Reno's vibe route, to keep it more balanced.
Same for Iharu's expression: the almost painful bliss of orgasm, of course, but it's heightened by the emotional aspect: feeling so wanted, being held and cherished so intimately ****. It can overwhelm, even trigger defensiveness/rejection; maybe Iharu would struggle to accept that kind of love, maybe it would, in that sense, be painful to experience.
Based on canon, Reno's love seems so deep and ardent it'd soothe any insecurities related to self-worth — and while it's not healthy to be reliant like that on somebody, it's nonetheless part of how Iharu and Reno fit so well, making up for each other's deficits/struggles.
• the way I colored their erogenous zones (well, the "genitals" part of that; there's also nipples) — in the explicit version — is also a reference:
mars (Reno's planet) is associated with sex and the erogenous system. So, even though Reno's bits are partly hidden from view and in the shadow, I made them more of a reddish/pink color, to represent that connection to mars, but darker/not bright, since he's quietly intense and not bright/etc like Iharu.
Iharu's, meanwhile, are supposed to match his more golden skin tone (that we see in the anime. It's funny how it fits: Iharu's is more golden, Reno's is more pink/red) and are more bright to reflect his bright, loud personality.
also, I've colored Reno's parts like they're in shadow, not for realism but for symbolic relevance: it IS relevant to show what they're doing, but Iharu's openness, freedom, and a sense of comfort/safety fit for showing him fully, while Reno's part of things is about intimacy in the sense of his affection being for the space between them and not for a show. It also makes sense with his personality and the intimate/reserved aspect of his fire (he seems distant and cool, might not care easily, but once he does care, he cares with all his might: devoted, obsessive, protective).
The theme also ties in with what I wrote elsewhere, about their likely dynamic in sex: Reno soothing Iharu's insecurities, Iharu letting Reno's fire burn freely, not some stereotypical submissive bottom but an active participant, a lover that challenges to burn together, emboldens to do that through his sharp, playful, bold presence.
* As a matter of fact, the two characters Reno has any close relationship with in canon (Kafka, Iharu) are both leos (the only ones in kn8, on top of that)=the heart(=the "music" body part, and Reno loves music in general). Whether that's an intended connection (or as in, the reason for Reno's love for music) or not. His love for shonen manga seems to stem from a desire to soothe a yearning for heros that could have saved his family from the kaiju catastrophe that killed them; then cooking and music seem random, but maybe they're not.
** Him sucking Iharu off would be an even better visual analogy: taking Iharu's "heart" in his mouth (aries is associated with the head), almost as if swallowing it so they're "literally" heart-to-heart sort of thing (kind of stretched, but why not, symbolism is all about that anyway). But I wanted the "clutching at his heart" element, so had to choose.
*** In the "passion" drawing I originally intended for both of them to be naked, but I decided that Iharu being clothed highlights Reno's vulnerable aspect — and Iharu's honesty and openness is still represented through his body language and facial expression. Still, him being naked as well would work perfectly. So, here we're back to that symbol: his honesty as his open body language while completely naked. Taking it even further from the previous drawing. He's not hiding anything, even when he does "have something to hide" [because it's not selflessness etc culturally praised things].
**** The peak of that would be chest-to-chest, since turning to something with our back is the opposite of an open body language, and of course of intimate (also of "feeling safe", though on the other hand one wouldn't turn their back to perceived danger, so it's less of a stiff association). But, again, for the sake of the heart thing being doable I chose this position. The fanart is symbolic, so the priority goes not to what makes it sweet (e.g. eye contact, hugging, kissing) (or sexy, for that matter), but to what best fits the idea I want to depict (too bad though, and I want to draw more intimate fanarts for them).