Oh man, that's when I felt that things started getting really, REALLY good. The ways that Stain's ideologies intersect and conflict with Shigaraki's is one of the things I find UNBELIEVABLY well-designed about the series and how and where the conflicts around the idea of a hero-filled universe would come up. There's a lot of depth all over that, and I think that as soon as Iida has to deal with his brother's injury is where the series feels like it properly cracks open from the more classic WSJ arcs and keeps playing with its tropes in really interesting and often unexpected ways.
There are a LOT of moments like that where the character development is SO damned good, because Kohei Hoshikori always seems to keep a lot of elements and character-specific motivations boiling behind-the-scenes, and then pops them up in ways where the characters feel a lot more like real people – especially people their age and how they can vary in how they express or internalize situations that they've had to deal with.
As a non-spoiler example: One of the in-betweener pages in the manga, he mentions one of the news casters we see in the background on tv has a quirk where he has two horns. The character removed part of his right one, so that it didn't obscure videos edited in behind him when he was reporting, but as a result he's received backlash as it seeming like he was making a statement of being ashamed of his quirk.
That's all stuff he's thought through for a character who doesn't REALLY have any lines of his own. It's why I think that where the series really starts to shine is knowing that Kohei is internally keeping track of character motivations of his characters VERY specifically. That when he uses moments and things like that, they feel REALLY fully formed, and he varies between showcasing those things front-and-center in the story, and having them happening passively, and only coming into the forefront when they're necessary as a point that it breaks into the story – like with Iida and his brother.
Super hyped for you to get wrapped on Season 2 & move in to Season 3, man! (Also, I think you'll quite enjoy how S3 uses that character you're into)
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Thoughts?