Whenever I dealt with new characters in the past - (from my time roleplaying and writing short stories at school) - I think it’s fun to do like “get to know them” mini stories that serve the purpose of you getting into the “groove”! They can be things added to the real story later on, but sometimes starting with mini-stories about the characters - (like filler type chapters) - can help!
It’s also a fun way to really describe their personality type and work with it in ways that can inspire you to really understand them.
New characters are difficult! ; o ; )/ Good luck!