strength always seems to be linked to the ability to stand alone, which is why i think so many ppl overlook atsushi's growth
in the beginning of the manga atsushi is filled with self doubt and the second he thinks he's bringing trouble to the ada he turns and tries to leave and go on the run
now mind you atsushi has just felt for the first time something that could be acceptance (say what you want about dazai's blackmailing, but has anyone wanted atsushi enough to try and trick him into staying? not the best way, i know but in all its um dazai-ness, it did show that atsushi had something (his ability, etc) that was worth having) and the second he thinks he's hurting them he tries to leave --- he doesn't ask for help, he doesn't try and talk it out, he stands alone
and i dont think atsushi's weak for doing this, mind you. atsushi's kindness and selflessness are both things that are part of his character, and i dont think the headmaster had any part of it, despite what atsushi believes --- but for all it shows atsushi's care, selflessness, and kindness, it also shows how little he values himself and how much he believes he's essentially unimportant
(editing to add: it also shows his inability to trust the ada's strength overall, and their commitment to their members, and his own role as a part of them)
him leaving shows that atsushi has always been alone -- at the orphanage, with the headmasters "attention" he was alone, in his suffering, his pain --- or perhaps its that he's alone because the headmaster follows him forever - becuz in a way he's surrounded but yet somehow he's all he's ever had
when in 55 minutes atsushi tells kunikida and dazai about time travelling, its not a show of weakness it's a show of growth
it shows that atsushi has learned that he doesn't have to be alone. atsushi can rely on others --- even if atsushi still thinks the worst of himself, he's starting to think of himself as part of the agency -- as someone who can ask for help
strength and growth arent only the ability to stand alone --- which mind you atsushi has done -- he's done it for 18 years and he may not have lived but he fucking survived with all odds against him, he survived and with all the makings of bitterness and anger, he did his best to be kind
but atsushi doesn't have to be alone -- humans aren't meant to be alone, he's better when he's with people he can trust and with the agency he's able to have that and he's able to understand that he has that
atsushi's character goes from someone who only has himself to someone who can rely on others --- someone who can be strong becuase he has people, and who, when weak, still has them
this is pretty much explained by atsushi himself on the boat pre-akutagawa
atsushi has always been alone. but now that he's tasted what not being alone is he doesn't know how to be alone anymore -- not because he's weak but because he's grown as a character, because he's changed
because he knows that he can be around people who won't hurt him
atsushi fights stronger when he's with akutagawa because strength isn't being alone, it's the ability to trust others -- despite only learning why you should never -- it's the ability to reach out and stand next to someone as their equal and fight together
atsushi is easily one of the strongest characters, because he's finally learned he can rely on people
his character growth and development are breathtaking to watch