Martin Blackwood never really changed.
His attitude changed, sure - he went from "everyone I care about hates me so I must deserve it" to "I am terrible but so are these people and since they've been so mean to me I should take revenge, I'm not as weak as they all think". But his way of thinking stays the same throughout the whole series.
What I dislike about Martin and what stays the same even after his "character development" is that despite claiming to be a carer, he consistently fails to consider things from other people's perspective, and his fans overlook that part. I've always felt like his attempts at caring for people (bringing them tea etc) were somewhat clumsy, and that he never actually knew what they even wanted - i think he said so himself when he was in the Lonely that giving people tea when they seemed down was easier than discussing their problems and actually finding out what's wrong. It shows when he just jumps to sacrifice himself when Peter Lukas took over the Instutute - he was so quick to sacrifice himself and give himself away and he didn't even stop to consider that it might be unnecessary, or that there might be another solution, like discussing with his team. His help is less about actually helping and more about punishing himself and not being useless. Since he hates himself, that kind of help is not only "making up for the space he takes up" in his mind, but it's also kind of like self harm.
It also shows in season 5 when he straight up refuses to listen to Jon's explanations of the apocalypse and the statements in general.
Despite being so set on sacrificing himself for others, berating himself, and generally always trying to fix everything for everyone by himself, Martin has only ever considered his own needs.
It actually makes him feel very real as a character, but unfortunately most of the fandom has dumbed him down to two versions - season 1-3 "cutie shy softie boy with a crush on his boss awwww" and season 4-5 "hot sexy man that wants to take revenge on everyone who has ever wronged him and he is so in love with his monster boyfriend uwu❤️❤️ jon and martin are so cute together ❤️ " no they're not. Maybe Martin was fine with Jon before the apocalypse - admittedly i don't remember the safehouse ep all that well, and that seems to be what got everyone to ship them, but after the end of the wold, when they're travelling through the Fearscape together, they are so very not happy and healthy. There are so many problems. And Martin straight up refusing to listen to anything that Jon has to say about the Eyepocalypse is just the cherry on top.
Towards the end of the story, they use each other more as anchors that help them keep the remaining bits of their humanity. They ground each other, true, and they NEED each other, but their relationship is messy and tainted with the feelings of guilt, loss, and blame. They barely communicate their feelings and intentions. They stick together in order to survive, but the only thing bonding them together is shared trauma. They are broken by what they experienced. They actually don't know each other at all, just the idealized, pre-apocalypse versions of eachother. It makes me quite upset that the vast majority of the fans missed that very important aspect of their relationship.
It was literally spelled out for them by Peter Lukas:

So yeah, maybe they are canon in the sense that they were briefly in a "romantic" relationship. But the fanon version is sooo romanticized and warped and twisted and changed and made to fit into one of those neat "workplace-enemies-to-lovers" boxes like tags on ao3.
To sum up: i don't like Martin as a person, but he's written well and realistically as a character. I despise fanon Martin and Jmart. Jmart is canon, but not in the way most people seem to think.
ily xoxo you’re always right this is why you’re my favourite mutual (alongside all my other mutuals. you are all my favourites.)