Thank you YuSePe! Yes, I looked at every tutorial I could find on youtube for beginners on Paint tool Sai. With your aptitude with colors, you will surely make a great use of gueuzav's ([link]) tutorial: [link]
You're right, I thought about wow's draenei the first time I saw soraka in league of legends: blue skin, horn, horse legs... This one took me a lot of time, it's my second work with Paint tool Sai and I tried many things to learn and experiment the new tool I spend, maybe one full day of work from the first sketch, two days including breaks, making other things and sleeping at night... wait, the weekend is already over ?!