I think So-Ah shouldn't ask "who" but "what". It's not like Tenka fell for Xion at first sign but is more about his carefree traits. He showed her how simple life is unlike how stuck she was before, she never actually enjoyed her youth since she had to get rid of her "flaws". Now Tenka changed her prospective a little bit and is being more carefree and relaxed, exactly what So-Ah isn't because of all the pressure on herself.
I think So-Ah shouldn't ask "who" but "what". It's not like Tenka fell for Xion at first sign but is more about his carefree traits. He showed her how simple life is unlike how stuck she was before, she never actually enjoyed her youth since she had to get rid of her "flaws". Now Tenka changed her prospective a little bit and is being more carefree and relaxed, exactly what So-Ah isn't because of all the pressure on herself.