I have literally no idea how this matchup works, and I need help to know 
I know that the Counters are a pain…
Yeah, that matchup is garbage lol.
Can’t help you.
Is that true? Riptor is considered his toughest match up?
Not Arbiter’s worst matchup but definitely not in his favour. It really depends on the hisako your fighting (constantly uses counters, pokes , command grabs a lot) but carbine is great for footsies and grenade pressure can help limit hisako’s offensive or bait out command grabs her counter hits and better framed normals are what really take the cake for hisako imo