Here's what you need to know about To-Love-Ru:
This is Yuuki Rito. He's a boring high school student and works part time as a chick magnet. Literally.
Aforementioned chicks. Going from top to bottom, left to right.
Mikan: His (younger) sister. Perfect housewaifu. Wants Yuuki's dick but denies it.
Kyouko: Star of a magical girl show. May actually be a magical girl. Wants Yuuki's dick but is hiding it from Ran.
Ryoko (with the chest): Slutty alien nurse. May also be bisexual. Wants Yuuki's dick, but only because she's a cocktease.
Oshizu (in the back): Ghost of a girl who was given a body by Ryoko with her alien magic. Has psychic powers. Afraid of dogs. Does not actually want Yuuki's dick.
Yami a.k.a Golden Darkness: Morrigan minus Soul Fist. Has a taiyaki fetish. Best (lesbian) friends with Mikan. Wants to assassinate Yuuki's dick.
Celine (top left baby): Born from a plant monster. Likes breast milk. Does not want Yuuki's dick don't even think about you fucking sicko.
Ran (with the horns): Alter-ego of Run, who is a guy. They share the same body and switch personalities (and the body's sex changes to match) whenever they sneeze or some shit. Hilarity ensues. Ryouko's coworker. Wants Yuuki's dick.
Haruna (black hair): Childhood friend. Based on the author's wife who divorced him some time ago and jacked all his lewd manga money. Wants Yuuki's dick but is too shy to admit it.
Lala (pink hair): Genius alien engineer. Is engaged to Yuuki's dick.
Lesbian Trio: They grope each other and the other girls for fanservice and comedic relief. Does not want Yuuki's dick.
Nana (twin tails): Lala's tsundere sister. Has body image issues. Yami's other best friend. Talks to animals. Recently learned that she wants Yuuki's dick.
Momo: Lala's cool younger sister. Talks to plants. A known Mormon who supports the idea of Yuuki having a harem. Probably the closest one to Yuuki's dick.
Yui: Tsundere uptight class rep. Wants to replace the stick up her bum with Yuuki's dick.
Maron: Haruna's pervert dog.
Lesbian Duo: God I don't even remember these characters.
Other "girls":
Riko: Yuuki with a vagina thanks to one of Lala's "inventions", but no one knows it's him. His best friend wants to get in Riko's pants. As you can imagine, this is kind of awkward for Yuuki.
There, now you know everything you need to know to start reading Darkness.