You people really like to torture me with difficult questions /lh ... the thing you have to know is I tend to gravitate towards protagonist/主人公 archetype characters even when they're not the Actual Protags. I'm also very fond of underdogs' stories, and Ace got all that. The first time I saw her racewear drip and that smug G1 winning pose, I immediately went on a deep dive on the IRL horse and what did Cygames infer from the real life horse to render and write the Umamusume version like That. Because like, she's so cool.... is the IRL horse this cool too? (He was, in fact, cool and cute).

I read the IRL history first before jumping into her ikusei, so of course my expectation in how they would execute her story was high. It's very safe to say that I'm very well satisfied with how they handled the story, but that's a given because it's Umamusume. The bigger factor was how her preconceived personality—rambunctious, rebellious, reliable, strong willed, always too focused in marching forward that it almost sabotage her relationship with her rival, easily frustrated and isn't shameless to express/admit it—plays very well when incorporated into the real life lore. This what makes me like Ace the most ... :D

TLDR, to answer your question, there are three main things as to why I gravitate towards this nugu ass horse(girl):
1) Her shujinko design and personality. It's not helping that some people affirmed my delusion that her colpal (minus the trans flag colors) resembles Joker P5
2) Her IRL counterpart history... umm waiter waiter I'd like to order a TRIPLE CROWN KILLER who is also very cute please
3) The most important: CV. 藤原夏海 ^__^