Because I could see Tyee, AKA "Big T", becoming another favorite OC. Why? Because my damned affinity for big, bulky characters knows no bounds. ANYWAY, T is NOT meant to act as a babysitter during his working hours at Thornewood. Thus, Ryan will often come and sit for the kids even during T's time on the clock. Since T is about two years older than Ryan (18 and 16 years old respectively), they never got to be acquainted with one another in high school. And since Ryan and his family moved to Graves Island 5 years prior to the story, there's a chance these two NEVER met at all. This makes for a silly misunderstanding to take place shortly after T's first week on the job.
Ryan is often allowed to come and go as he pleases from the Castle, being given the house key by Uncle Morton so he can come by on a moment's notice. Unc and Auntie went out for the night and called Ryan over to the house, without informing Big T. As mentioned, T is NOT meant to be the babysitter; but if he notices that Uncle Mort and Aunt Claudia are away for longer than anticipated, he WILL take care of the kids and guard them, fiercely if necessary. When Ryan arrives, he doesn't usually ring the doorbell and just invites himself in, since he has the key to the house. He will usually go in the kitchen to raid the fridge first and foremost. So you could imagine how scary it was for T to walk out of the pantry and find a stranger in "HIS" kitchen. Ryan tries to ask who he is, but is quickly thrown against the fridge and held there in T's powerful grip. T, both terrified and angered that someone was able to slip inside the house right under his nose, begins interrogating Ryan. "WHO ARE YOU AND WHAT ARE YOU DOING IN OUR HOUSE?!?" Ryan is confused by the "our" in the sentence, having never seen this guy in the house before today, but can't really ask T any questions since he's got him practically in a choke-hold against the fridge. The kids hear the shouting and come down the kitchen steps to see what's up. They explain to T who Ryan is, T sheepishly apologizes to Ryan, they shake hands, and become pretty close friends.
So, to wrap it all up, Tyee is basically both the household Cook and the kid's bodyguard when no one else is around to do it. (Ie Ryan.)