That's why I have a problem with Mayumi here. There's a slight gay between his attitude in the previous mangas and here. And I'm not talking about him growing out of his shell or something.
Here he acts as if Taiga abandoned him whereas in the previous one, he was feeling guilty for exactly the opposite of it.
Everything that made me love Mayumi as a character, was destroyed for what at one point just seemed like a temper tantrum. It feels like the author had an idea at first and the completely changed it midways.
but he is just a kid, you know. Taiga is like a mother to him, so it's normal to feel like Taiga abandoned him. suddenly there a guy who Taiga care so much for him and love him, and he spent almost his time with, even a kid would feel like that if something like happend, to think what happend for him in the past, that's made a strong bond between him and Taiga, so ofc. he will feels like that. He is the baby in his brothers eyes, and he like to stay like that.
I love Mayumi and his story, but I will lie, if I not say I was upset when he began to grew up (when he spent more times with the other kid)
but well im my eyes he grew up well, I don't have problems withh his behave or changed in the character ? but sometimes I miss when he was his brothers baby.
sorry if my comment is senseless, im very bad to explain in words.
I'm not saying that it wouldn't be normal such a situation. Nor that it's bad that Mayumi is growing. Is just that in the previous volume, he was growing in a very different direction and he was the one feeling guilty for abandoning Taiga, not the other way around. In the previous volume, he felt guilty for not being there for Taiga and for not being the one to save him. It wasn't jealousy, insecurity or anything. It was guilt. He was a really well thought character. And now he's jealous of Shuu and feels as if Taiga abandoned him? If you consider the last two volumes, it doesn't make sense.
mayumi in the previous story had too feelings, he never wanted to leave Taigas side, however, he learned that those feelings and taiga guilt were tied together, so he tried to independent outside of the house and a baby in the house. However, he could not be a baby in the house any more because Taiga's No.1 was Shuu not Mayumi. SO mayumi was becoming "unstable" because he wanted to remain a baby in Taiga eyes, but he already was outgrowing that and being replaced with his brother's lover. SO in his eyes, Taiga "broke" his promise in order to be with the one he loves. This is no different from the last volume, you simply forgot there were 2 things Mayumi was struggling to reconcile. Growing up or staying a baby.
Stop talking about Mayumi like a bad person. He is one of the best character here.
even with his sad past, he always smiling and will not have Taiga feels guilty of what happened in the past. and even when Shuu come and take Taiga away still he smile for his brother sake. He is so strong and so kind and think of his brother before he thinks of him self.