You need eccentrics for a story that focuses on quantity rather than quality because it is quite impossible to make layered characters in this type of story; it requires too much time to develop to which they won't have. Eccentrics however are the opposite; very easy to develop, not very layered
What are you even on about..? All of the characters have layers.
Hakari was raised as some rich kid, yet never acts better than anyone else (except Karane)
She treats Mei like a little sister despite her being a maid for the family because she originally wanted her to be her sister,
Karane came from a line of super tsunderes, yet fights against it.
She's also a gal, which doesn't even come to light until Ahko recognizes her style as a fellow gal and takes her on a gal date.
Shizuka seems weak but is incredibly strong in spirit, kind, and forgiving.
despite getting slapped for being different by her own mother she still clung unto her book AND didn't even hold a grudge against her mom.
Nano seems like an inhuman android, yet is often the first to share her feelings (and has had consistent character growth in understanding people, to the point of being able to befriend Mimimi on her own.)
Yet she doesn't always make the best choices, as she's seen failing on multiple occasions such as the soaking maid party
Kusuri seems to love drugs more than anything, and just be sort of a mad scientist, yet it's later revealed that the immortality drug wasn't even for her, it was because her grandma was going to die of old age soon.
Despite the risk of never being able to make drugs again (or at least not very impressive ones) if she really did get trapped with the body and intelligence of an 8 year old forever she still conducted the experiments along with the help of her parents.
We also can tell from the little glimpses we get of the 18 year old version that she's actually pretty silly and immature, even as an adult, the only thing that really changes is she gets embarrassed over different things.
Hahari has a love so extreme that she's completely blinded by it, to the point of risking death just to have a kid with her lover before he dies, not even thinking about the fact that would leave her alone to raise the child, yet she respected the boundaries he set and didn't kiss him even though there's not much he could have done to stop her.
Kurumi is always hungry, which is seen as "haha funny anime girl eats so much heehee" by a lot of people, but she's actually one of the most tragic characters. She's (almost) always starving, likely to the point it's painful, and yet despite being in a near constant state of pain, she tries to minimize the amount she lashes out at other people, and takes the time to take off her headphones before eating.
... So do you want the extended list, or is this enough characters and depth for you?