Guest :Hi... is Kate?

John :Come on in! Kates in the kitchen. Im John... Kates boyfriend.

Guest :Im so glad to meet you. Im Elena, and Im an old school chum of Kates. Oh, what a comfy looking armchair! Has someone taken this seat?

John :No, be my guest. 

그냥 무심히 지나치다가 궁금해진 건데.. 왜 Kate 에 s를 붙여서kates로 쓰는 거죠?  그냥 im an old school chum of Kate가 아니고...