@Renard
@FStubbs
Lol, what are you guys on?
She's clearly a side character that the manga's currently focusing on for now. That's like saying the Zoro arc in one piece made Zoro the protag.
The only person it's been hinted at that Ai likes is the MC. MC's the only one that's shown to be kind and supportive. MC's the only one that's charismatic enough to actually draw people to him. Not saying Yukiko's a bad character though. Her pragmatism and bullheadedness make for an interesting character. Definitely not the lead character, and I would honestly be shocked if she makes it out of this predicament on her own without the help of the actual MC and the child genius.
@djamel90
Hounds aren't important to the main story. They're only important to this villain's character arc so far.
Ai is still a mystery but the reason she's so important is because she's the catalyst. Confessing to Ai is what causes the eye in the sky to appear and time to turn back after Ai gets brutally murdered.
@Tamerlane
If you want good harem romance stories go read something like Mushoku Tensei. He actually marries 3 different girls in that one and is able to maintain a happy family life for a while.
Isekai Nonbiri Nouka goes a ridiculous route and the MC has over a thousand romantic partners, but that story is kinda nuts.
There's also Parallel Paradise, and Shinju No Nectar.
All the main storylines for those manga are actually really good too.
Personally don't mind those stories as it's just wish-fulfillment stuff, but I disagree when it comes to this manga in particular.
Since the MC doesn't like Yukiko in that way, and Yukiko doesn't like the MC in that way (for obvious reasons,) I think it's better that they're competing for Ai's affection. It's also a show of their friendship that they're able to put that part aside so they can help Ai. I think the current dynamic makes them more interesting as characters.