A Sensei is like a teacher. Our Sensei, Splinter, raised myself and my three brothers, so he's not just a teacher, he's kind of like our father. He taught us everything we know, and he's very patient with us. He nurtured our individuality, even though the four of us are the same age. I hear a lot of parents don't do that with their kids (especially when they're multiples), so I think we were really blessed to have him.
Oh, by the way, my name is Donatello. My manners slipped for a moment there. I'm just really surprised to know that fairies really exist! Though if we were brought from a different reality than each other, I'm sure that explains it. With infinite possibilities, there's infinite realities, and everything would exist somewhere.