Elena Narahashi
She is an extremely outgoing, playful and friendly person, and her nature has been described to even surpass Yuki's mischievous little sister mode, often making lewd and suggestive jokes. She shows herself as a rather eccentric character, but she does have a more mature and earnest personality, and as Masachika says, she's actually quite shy, sensitive and a fundamentally sensible person, but that side is mostly kept hidden away.
