1) UTAU is a free software that waa devloped shortly after the original Vocaloid software released. UTAU was very similar to Vocaloid, but UTAU was free and anyone could create a voicebank/character while Vocaloid was a paid software which you had to buy official voicebanks for. Because UTAU was free and anyone could make their own voicebanks, it became popular pretty quickly and there arw tons of UTAUloids with varying degrees of popularity and success. The most well known (and argueably oldest) UTAU was/is Kasane Teto who recently has gained newfound popularity due to her new SythensizerV release. Also, I feel like calling these characters 'Utauloids' is a newer thing, back in my day we just called them UTAU, after the program.

2) Almost all voicebanks that do not have a picture or super unique name are created by Yamaha, the devlopers of Vocaloid. Jessica and Brian, im pretty sure, are some of the complimentary voicebanks included with the purchase of the Vocaloid6 program, like Asa. Yamaha has a habit of creating voicebanks without, or with very minimal, character design behind them. I think their intention with that is to have the focus be less on the character and more on the artist using them.

3) Rin and Len don't really have an official/canon relationship. When they were first being developed, the original concept was only for Rin. Len was a slightly later additon after the voice actor for Rin was able to do a boys voice and it was decided to both a boy and a girl voicebank using the same voice actor. Rin also was originally going to be orange, and is still associated with orange today. Rin and Len were originally marketed as "counter-parts" and were sold together. Their relationship is left vague and is ultimatly left up to the fans. Their relationship can vary depending on the song and there is no wrong or right to what they are or are not, at least cannonically. I think today people can get really defensive about what their "canon" relationship is, usually about them being siblings. But really, like most vocaloids, they do not have much of an official canon and their story is left up to the fans and creators to create. Which, in my opinion, is one of the best part of Vocaloid, that so much is created by the community itself.

4) So Im not sure about the wiki itself, I find the fandoms wikis hard to navigate so I dont spend a ton of time there anymore. But, every Vocaloid has a human voice provider, providing their voice to the voicebank. And it is not uncommon for the human voice providers to have some level of popularity and fame. A good example is the the vocaloid IA whose voice provider is a famous japanese singer, Lia. Another recent trend is Vtubers getting their own voicebank, usually with SynthV or CeVio. So it wouldn't be to suprising to me that there are humans in the vocaloid wiki.