As far as characters I grew to like:
Jakob - Honestly I was originally planning on marrying Silas in my first BR play through but I ended up caving to the master servant relationship like I always do and I don't regret any of it. It helps that I like him as a character.
Arthur - All I had seen of him before playing conquest was his character portrait and for whatever reason I hated how he looked, and was pretty dead set that I wouldn't like him. Granted, I had no idea what his personality was like. When I finally got around to playing conquest he ended up being one of my favorite characters personality wise. It's too bad I had to bench him because of trust issues.
All the Nohrian royals - I just kinda felt like I would hate them because I seen them as the douchebags that stole me from my "real family", but again, once I played Conquest, I grew to like them a lot as all their personalities really came to life and made me genuinely feel like I grew up my whole life with these guys as my siblings.
Takumi - His bow is stupidly good and his crit quotes are music to my ears. That and Conquest.
Unpopular opinions inbound, characters I started to dislike:
Almost everyone from Hoshido - It's not that their bad characters or anything, I just found them all really bland and boring. None of them stuck out to me like the Conquest characters did. ESPECIALLY the Hoshidan royals besides Tacomeme, holy cow were they boring and uninteresting. Ryouma was nothing but "i'm a good guy that does good things" at the very least he was a badass unit. Hinoka was tomboy to the max, and Sakura was doormat/wallflower the character. And none of them really ever changed in my eyes. They got a little more interesting on Conquest but still just...bland. And it's sad because pre-release I really liked them and were the main reason I chose Birthright first.
Felicia - I don't understand her appeal. Her voice is a bit grating and she's just a klutzy maid.
Azura - Ohhh Azura, I thought I would like you, but you ended up being another quiet, introverted, borderline marry sue that has like...1 or 2 cool scenes but in the end is just there to be the one that knows everything but can't say anything because "you need to buy the last story". She also shoves "Lost in Thoughts, All Alone" down your throat in Birthright which is a good song, but it gets really tiresome to hear after a while. Not a bad character and I can understand why people like her, she has a nice design and her voice, while she's too quiet, is still a good voice, but at the same time she just kinda irks me.
