Generally, matchups are considered in sums of 10. So when you say Kohaku vs. Ciel is 3-5 in Ciel's favor, you probably mean 3.75-6.25 in Ciel's favor. I'll shorten it to 4-6 which sounds about right.
There's already a matchup chart made by Arly. You'll find it here:
http://www.meltybread.com/forums/index.php?topic=1170.100Since I usually play KohaHisui pair, my experiences differ from yours a bit, but it shouldn't be that much. Just off what I remembered:
Kohaku vs. Arcuied: 5-5 sounds about right.
Kohaku vs. Akiha: probably around 3.5-6.5 for Akiha. Kohaku doesn't deal enough damage to Akiha's GGPO endurance to compensate for the damage Akiha can do to you.
Kohaku vs. Nero: probably 6-4 Kohaku's favor. You get FREE instant overhead and Nero teleporting from plants is just bait.
Kohaku vs. Sion: probably 4-6 Sion's favor. You get more health as Kohaku solo but not enough to deal with Sion.
Kohaku vs. Miyako: lost me there. I don't play enough Miyako. I would guess it would be around 4-6 Miyako's favor though.