Last night I beat the game and got the A ending.  The thing I'm wondering is why did I not give TK the Zombrex?  I had it when I beat the game but it never acknowledged it.  The Zombrex I have is the one I bought at a pawn shop.  Did I actually need to go hunt him down before I beat the game?  And if I did were is he?  There never seems to be an arrow for it.

If you give him the Zombrex, you will receive Ending S, which is the 'best' ending for the game, as far as I know. I don't know where TK is located though, due to the fact I don't even own the game and just happen to know information from friends.

You kind of put a spoiler in the title.

Yea I know thats how to get ending S I just don't know how to locate him.

If you give him the Zombrex it opens up 'Overtime Mode' which if you complete gives you Ending S. 
You actually have to go into the medical room and give him a dose like you would Katey, having it in your inventory alone isn't enough.

If you don't mind spoilers 
 

" @InfamousBIG:  Shit your right lol I don't think there is anyway of changing the title!!
 
@SteamPunkJin: I didn't realize he was going to be back at the room with Katey, I thought he ran out into the mall! "

Yea finailly found him, I thought he left the safe house!  It would have been nice if microphone lady could have told me he was back there!  I ran around looking for him thinking eventually she would tell me where he was. Because I thought he left the safe house I figured why go through that un-needed loading screen.
 
@SmashedControllers:
Yea finally went through all that stuff, now I just have to beat TK in the arena later tonight.

" @SmashedControllers:  Hah thanks a lot.  I'm definitely not doing that now, boss fights are so tedious. "

"You kind of put a spoiler in the title. "

Can I give TK the zombrex during the part where you're chasing Sullivan, or is it too late by then?

i give TX the zombrex so i can get over time if you beat over time you get S ranke