I think that's saying too much, Seinaru wasn't bad at all but she definitely had... I don't know how to quantify it, but she had some "OC writing syndrome" to her- not near as bad as a lot of the other guests, but it was definitely there.
Personally, my favorite guests were Roxanne and Harumunia, though for different reasons.
Roxanne to me felt the most like an official character, minus the shrinking powers, but I loved her size complex and how she'd tease you, give you just enough to leave you wanting more- where her route ends with her saying something like "maybe if you continue to be a good servant, I'll reward you more" I tell you I'm not into feet and I would have licked every inch of her toes and inbetween at that if it was what it took
As for Harumunia, she's got that casual lewdness and teasing down pat that I love in a giantess- she's "sweetly condescending" where she always refers to you as Tiny, and how she comments that she loves "playing with" tinies, but at the same time isn't a psycho bitch who sees you as nothing but her entertainment. Her design though... Leaves a lot to be desired, and some of her CGs especially suffer for it.
Despite what I said about Roxanne, Harumunia feels a lot like an official character as well, compounded by the fact she appears in the lineup of characters on the front page image.