Unknown Player on Account/ Advice

[sg-zephyr] For the past two days I've had a mystery player on my account, It started about two days ago when I noticed I had exactly 6 games that where played when I was sleeping. Tonight I went to bed a littler earlier and woke up around 3AM. My GF was sleeping so I jumped onto find that my account had been logged in on another device and was CURRENTLY IN GAME. I've been monitoring it all on OP.GG and logged into my account right after a game and was immediately put into a party with mystery players duo partner. Who ever's playing obviously mains ADC and has a relationship with his duo. I looked up the duo partner and he has barely any games other than games duo'd with my account, the mystery players duo partner has very few games otherwise. I changed my e-mail information so I can maintain control over the account but is their any other way he/ or she may be able to take control over entirely? I called my friends who play League and they where all sleeping so I have no idea/ have relationship with whomever is on my account. Does anyone wanna help me find the identity of this mysterious player?

(EDIT) THEY'RE STILL PLAYING!! But I'm almost in my silver promos [zombie-brand-clap]

(EDIT2) They got me to my Silver promos and quit for the night. I'll update tomorrow night.