What is carved into Toko's leg?
The mark seen on Genocider Syo is the Japanese Kanji 正

Wiktionary has a page on this character explaining its use as a tally mark by the Japanese. 

Genocider Syo uses the tally to count the amount of victims she has killed. There is also a small section on this on the Wikia. 

Since each one represents 5 kills, you can see she has killed quite a few!