Len appears in the real world as a virtual singer. When young people have trouble understanding their true feelings, Len and his fellow virtual singers appear to help them. He is a child who was lost in the Sekai for a while before being found by Miku. He is very shy, often hiding behind Miku.
(Quoted from Sekaipedia - https://www.sekaipedia.org/wiki/Kagamine_Len)

For Len, we're looking for vocals that power and firmness to it, but can also excellently convey a wide range of emotions.
Please send me two clips: one of your normal, speaking voice using the lines provided and more if you want, and one of your singing - of any song Len is in.

Vocal reference: https://www.youtube.com/watch?v=LvKUWh3w5tE 
Speaking Reference: https://www.youtube.com/watch?v=3VLD6wKcoDE