Oy, I think I'm a little late but here are my headcannons~
Miku: Miku is quite an airhead. She's oblivious to a lot of things, especially when it comes to romance. She's also very curious and amazed by almost anything. Also, she's clumsy. No matter what though, she's smiling. Although she may seem like she has the mind of a three year old, Miku is very sympathetic and intelligent when it comes to something she's passionate about. She's passionate about music and wants nothing more than to make people happy with her songs. Miku is also a very good advice giver. When the not-so-popular Vocaloids are feeling down, not confident about if people like them, or whatever it may be, Miku becomes sort of like their older sibling. She lends them kind words that have a powerful impact, whether she knows it or not.t's very rare to see Miku upset or crying. She's always happy and bouncing off the walls with energy and a lightness to her that makes everyone smile. Though when she does feel upset, Miku bottles these emotions up inside of her, making sure no one sees it. Though when someone asks her about it, she can't hold it in any longer. Overall, Miku is generally an oblivious airhead that has a sincere, kind heart.
Rin: Rin is definitely the leader of the two Kagamine's. She's always thinking up plans and dragging Len into them. Her mind is like gears, always thinking. She looks up to a lot of the female Vocaloids, especially Miku, Gumi, and Luka, and sees them as older sisters, usually calling them "onee-sama." Her voice is nasally and sweet like honey. She gets easily agitated and can be a tsundere at times. Most of the time though, Rin is an outgoing, cheerful teenage girl. She loves adventure and is very curious like Miku. She is also very competitive. She absolutely hates losing. She's quite stubborn as well. Rin doubts herself though. She feels like she'll get left behind by everyone once more Vocaloid's come into the picture and she will be overshadowed by them and forgotten. She gets depressed at this thought. She loves her 'family' more than anything and would do anything for them.
Len: Len is the follower of the two Kagamine's. He's responsible and more mature than Rin. He's quiet, but very sincere. Even though he's younger than Rin, he feels the need to be protective over her. Len gets easily embarrassed. Rin uses this to her advantage to tease him. He knows she's only kidding though. Len has the same mindset when it comes to doubting himself. Also, he feels like he's just Rin's shadow. He goes to Miku, Luka, and Meiko for advice a lot. Like Rin, he loves his 'family.'
Gumi: Gumi is spunky, outgoing, clumsy, and overall just a complete ditz. She's curious and loves surprises. Also, she has a wild imagination and she's very childish even though she's almost an adult. Gumi isn't afraid to look silly or make mistakes. She's that person who laughs at herself because she did look silly or did make a stupid mistake. She's very adventurous and loves to do stupid things like a dare-devil. Gumi just loves to have fun. That's what she's all about. She cares deeply for those she loves and would do anything for them.
KAITO: Kaito somehow always finds himself getting in trouble by Meiko. He is quite a ditz though and doesn't think things through. He loves ice cream more than anything and finds himself getting bribed with it frequently. Of course, the bribes work. He's childish though sometimes he acts the age he is. Kaito is very welcoming and friendly and just has a warm vibe. He's looked up to by most of the other Vocaloid's as an older brother. Gakupo is usually seen with him. These two have a close friendship.
MEIKO: Meiko is calm and very mature. She finds herself "babysitting" Kaito. She gets embarrassed at the way Kaito speaks of them as a couple, or how cute she is ect, and beats him up because of it. She's a tsundere, that's for sure. She loves Kaito, yet she's in denial in front of the other Vocaloids. She calls Kaito "Bakaito," whenever he does something childish or stupid. This nickname comes from Baka, which is idiot in Japanese, and Kaito. Meiko wants nothing more than to be a mother someday. She's very unstable emotionally and turns to drinking to get away from these emotions. When she's drunk, she's outgoing and kind of scary since she's so crazy. You definitely don't want to get on her bad side when she's drunk.
Luka: Luka is calm, quiet, and kind. She loves to read and you can usually find her nose stuffed in a book when she's not singing. She's very mature, probably the most mature Vocaloid. She looks after the younger Vocaloids. She's usually seen with Miku, Meiko, and Gakupo.
I have many more buuutt I think I already did enough xD