It's actually quite hard for me to get attached to characters outside the main one or two. At least, enough for me to want more from them. When I do, usually, it's for companion series' that allow me to eventually experience a book with that character, so I did my best to name a few!
Kenji Kishimoto: this character was at the top of the list of my favorite things about the Shatter Me series. I don't think my love could build for him but it did with each book until the end. He was amazing! I would not hesitate to read a book about his own journey.
Maggie Carson: I really enjoyed the special epilogue that Colleen Hoover gave us, it intrigued me and I think her story would also make a good book in addition to Maybe Someday!! It also has the possibility to be even more heartbreaking and angsty than Maybe Someday, and I can never say no to that.
Lilah Montgomery: In Open Road Summer, we get a little peek into her life from the perspective of Reagan. But Lilah's ending is little bit bittersweet compared to Reagan's and It would be interesting to get an even bigger look from her side!
As a bonus, here are a few spin off series' from other great side characters with their own books.
Bloodlines by Richelle Mead - It's been a dream come true to get deeper into Adrian Ivashkov's character and to see him grow in this series. He left such a wonderful impression in Vampire Academy and I was just as hooked on him as Dimitri, maybe more.
Alpha & Omega series by Patricia Briggs - I really enjoy her urban fantasy series with Mercy Thompson, and I'm currently in the midst of finishing up to the newest book, so it's cool to get a peak at the pack her main character grew up with now that she's not with them anymore.
Made by J.M. Darhower - This was the most anticipated spin off ever for me. I was so obsessed with the authors character, Corrado Moretti, in her Sempre Series and was beyond excited we'd get a book about how he become the man he is in the other books!