Excuse me, if I can not formulate my feelings on why Chiyo is best girl in a clear and conscious way, English is my second language and as such, no matter how hard I try to formulate as best as I can, so I can come across with my point, I might still somehow butcher it.
With that out of the way.
For me Chiyo is best girl, because she is such a eternal presence gameplay wise for me. Especially on the challenge floors of the Spire of Courage, to the point where I don't even know to begin with building my party, without her in it.
Especially her strong AOE attacks with different elements that almost basically cover everything you need to wittle down a Boss's health. For me, Chiyo is just the best and most useful unit you have in game.
But of course, just because she is good gameplay wise, doesn't mean she is best girl. I understand that.
Here is why I like Chiyo personality wise.
She cute.
Now, now. Let me form my thoughts and opinions. I have difficulty doing this in english.
So, first of all. Chiyo is just generally a cute little Sorceress, trying her best to be of any support in the party. I guess you could call her moe? Like, despite her overall disconnection to the story, Chiyo just seems to be a fresh breeze from all the over characters that actively go after Ryen or are more assertive about their desires.
Other than the innocent Mira, who thinks Ryen is her destiny and barely after three scenes already fucks with him, Chiyo has a more gradual process, starting with exploring her feelings for Ryen, then a bit of kissing, before going a step further after each mission, until she is mentally ready to be with him and alongside that development with her relationship Ryen she also grows as a character/sorceress.
Like, she really tries her best to fit in with her new group of friends mostly harem mates though, working hard, so she can be a useful sorceress. I mean, at the end of her Storyline, she can even do Alchemy for free for you, without an alchemy table. Who is more useful from a gameplay perspective, than Chiyo?
And of course, her having a hot sister, who is the new ruler of Amagal is a plus.
But, before I stop trying to rant on and on about Chiyo, let's not forget the most important part.
She is the only real playmate to our cute dragon daughter Kayelinth and I really wish to see more scenes with the two together and her trying to reel in the chaotic excitement that she is. Like, the dynamic between is just so cute, I like to see more of it. Her being friend of Kayelinth already makes her worthwhile.
Now that I think about it, getting her is also really random, I mean she is almost getting raped in a basement by two Amagal dudes, considering that she is the sister of a rather powerful mage, I am interested in how something like that could happen.