How can I improve my characters?

I'm on the fourth book in my novella series, and the characters still feel shallow. I've spent a lot of work on them, but I can't seem to get past the "cliche alien" trope. Have any suggestions?

Here's a quick list of my main characters:

Marella, age 15, human, is occasionally emotionally unstable and usually reserved. She has an alien boyfriend named Denolde [nicknamed "Nold"] and lives with his family on the alien planet Sarr. She has a twin sister named Mandie [human] who also lives with her.

Nold, age 16, alien, is humorous and quick-witted yet compassionate. He is Marella's boyfriend [already mentioned]. He is the son of Tiergan and younger brother of Raesh, and his mother, Losee, was shot on Earth fifteen years ago [humans shot her, but no one can narrow it down to who]. He inherited Tiergan's confidence and sarcasticness, and Losee's wittiness, but not her intelligence.

Raesh, age 22, alien, is a recognized genius. He is known for being a skilled doctor, and is usually stoic and uncomfortable with emotions. He has many hobbies, including memorizing maps and novels.

Tiergan, age 39, alien, can be found recording music or saving Nold from some sort of danger. He is known for being a strong leader, though he can be brash. His wife, Losee, was shot by humans [already mentioned, sorry!], which drove him to insanity, but every day he gets better and has even found a new woman to love [her name is Lyrik, but her character won't be listed].

Jerome, age 43, alien, is Marella's best friend and emotional rock. He is Tiergan's older cousin and Raesh and Nold's second cousin. He is a skilled marksman but that didn't matter when Marella's human dad, Jake [not listed, he's too much of a brat to!], kidnapped him [he kidnapped Tiergan first and broke all of his bones, but then Marella saved Tiergan, and it's this crazy tangle of torture...]. He's known for being levelheaded and humorous at the right times [unlike Nold].

Obviously, there are SO MUCH MORE characters, but anyway, how can I improve them?