I've read enough yuri to be well-acquainted and rather exhausted with the 'secretly gay best friend who watches as her crush is carried off by a new girl trope', but Miko seems legitimately likeable. Maybe it's because she genuinely cares for her friend's mental health and actively takes measures to support and reassure her, whilst always respecting her boundaries instead of going all Manic Pixie Dream Girl. Whether she's secretly a monster hunter, someone who's had her family murdered by a mermaid, or maybe just secretly a mermaid/ human-mermaid hybrid, it'll be interesting see how she bounces off Shiori. At any rate, I hope she gets a happy ending and many rewards for being Buddy of the Year.
Also, did anyone else feel ridiculous amounts of sexual tension between Miko and Shiori? I wasn't rooting for either of them so much as I just wanted them to make out. Now that would be a great twist- our MC realizes that she doesn't need a designated Girlfriend(TM) to recover from depression, and can just flourish in a supportive group of friends, while Captain Emotional Support hooks up with Princess Undine.