Hmm, that’s actually a tough thing sometimes. Usually, I’d suggest rewatching/rereading/replaying several the source material to further understand canon characters. Gather as much as you can from the source material (e.g. source material has anime/manga/game/novel adaptations; delve into as many versions as possible). Or you can also wade through the fanfics for your fandom and see how they portray canon characters. Drop the ones that seem to stray from the original, closely observe the ones that stay really close to the original.
Otherwise, have a reviewer or commenter who is well-versed to your fandom. They can usually tell you what’s right or wrong with the portrayal of the canon character. If you’re from a tiny fandom like me, try to cater to non-fans and ask them to tell you what they think about the canon characters. Ask them treat canon characters like original characters in an original work.
For example, the MC of my fandom is a silent dude. If a non-fan commenter tells me, “He speaks so little throughout the chapter,” I’m keeping him in character. If a non-fan commenter tells me, “He’s so loud and noisy,” I need to find what’s wrong with my portrayal. Maybe he has too much dialogue, or something like that.