Undervalued characters. Hhhhm,
I don't want to say Ami was undervalued in Toradora but she was really the strongest character in the show and had much better chemistry with Ryuuji then Taiga and was a natural match for him as their personalities contrasted and lined up a lot. There was maybe more in common with them. Ami felt more human then the others, especially when we learnt more about her.

Kurumi from Kimi ni Todoke. She's first introduced as a villain and obstacle for Sawako to overcome, but as soon as that arc finishes we don't see her very much. She's very mean but after she tells a bit of backstory we start to sympathize with her. Her conflict with Sawako was the strongest bit of the first season, Kurumi didn't do very much after.

Megumi, Food Wars. C'mon this poor girl. One of the things that bothered me with this show is how Souma heroes it at every turn. Megumi has that one moment to prove herself and bro is like step aside I got this. Megumi totally got the shaft in this show.

Can't think of too many, I generally like all the well received characters in shows.