HI. these are great, thank you <3
(23) What are the defining characteristics of their relationship?
I am going to answer this from a perspective of canonverse post-kira because i the shape of their relationship changes substantially over time, and I think it depends on when they get together, too.
Near's steadfast, quiet devotion to Mello. He has seen Mello at his worst and defended his every move, even those that directly impacted or threatened Near.
Mello's lifelong fixation on Near. His envy and admiration. The way he can't hold onto his resentment forever, no matter how he tries. His need for Near's attention and approval. His need to be needed.
Their history. The fact that their life experiences are so unique and fucked that the other is the only person who will ever understand them.
Near's willingness to forgive time and time again. Near's ability to set boundaries, extract apologies, and reign Mello in when necessary. I think Near is willing to put up with a lot of nonsense, but he isn't a doormat by any means.
In a similar vein: Near's belief that Mello can do better. Mello likes to think himself damned, irredeemable, wretched, monstrous, cursed, etc. Near strongly disagrees. He expects Mello to try to be good. Not perfect. But better. I think he's quite patient with Mello and understanding of his shortcomings and low moments, but he does not act like they're inevitable. It's harder for Mello to stay on his "I'm irreparably damaged and incapable of being good, or even good enough" bullshit when Near sharply and clearly just does not believe that to be true.
(27) How do they say “I love you” non-verbally?
It is my strong belief that Mello has the potential to be deeply caring / nurturing (though I think his care / nurture is often either bossy or a little abrasive). In my mind, this has to do with his need to feel needed; I think in some ways he would prefer to be needed than loved, because it feels more secure. All this to say, I think Mello has the potential for tenderness, even if (at least at first) it's coming from a somewhat dysfunctional place (feeling like he has no value outside of what he can do for Near).
To that point: Mello shows his love for Near by meeting his needs. He knows which foods Near likes best, knows the specific brands, knows that the only raw tomatoes Near likes are the cherry ones, extra-sweet, and he knows the two or three varieties of apples Near eats.
Honestly, early-on, I think he tries to conceal all of this a little. He gets the exact right kinds of apples, peels and cuts them, and when Near makes an offhanded comment about how thoughtful it is, Mello acts like it was total coincidence that he prepared it exactly how Near likes. The next time it happens, Near doesn't make a comment. He just smiles and thanks Mello. Mello still gets embarrassed about it.
If he's not minimizing his little expressions of affection, he swings wildly in the other direction and acts like he deserves an award of some kind for excellence in boyfriending for THEE smallest of gestures.
As for Near, I think he predominantly shows Mello affection by seeking closeness. Once their relationship is established / secure, Near is not only the one to generally seek out physical affection, but usually the one to initiate sex. I think the combination of non-sexual physical affection and clear sexual desire/attraction from Near helps to alleviate Mello's "no one will ever truly want or love me" derangement. Similarly, I think Near indulges Mello's possessiveness to some degree as an expression of love (also because he's into it probably. sorry.); this is, I suppose, verbal at least some of the time, but it's not I love you exactly, so I'm counting it.