Sebastien, who is 21, prefers to play centre or right midfield and is noted for his speed.
He has, however, been frustrated at Monaco by the form of French international Ludovic Giuly, who has scored 11 goals in 19 games for the French league leaders, and is hoping to make an impression for West Ham - despite the battle for a place he knows he faces.
"I am glad to be here; I arrived on Friday and I hope to get up to speed very quickly in training," he says.
"There are a lot of good players here and I have already seen that on Saturday - plenty of good midfielders and a lot of competition so there will be plenty of work to do to get a place.
"I would hope to stay beyond the summer if possible; I love the English game, there is a lot of space, and it is passing football."
He is confident without being cocky and adds:
"I learned certain things at Monaco that I will try and put into practice here."
He began his first full week on Monday and adds:
"I met Alan when I came two weeks ago for a trial and I am looking forward to working with him."
Christian Dailly has had a glimpse of what he can do and says:
"He has got a bit of pace on that right hand side and there is good competition for place all over the park now."