Even If You Slit My Mouth (Webcomic)

Miroku-san, the slit-mouth woman, is to marry high school student Kouichi to maintain her existence as his family has been in charge of upkeeping superstitions for generations. To add, there seems to be another reason that he seems more than willing... 

Miroku, however, would rather use her own strength than borrow that of others. Thus, they establish a contract as follows.
1. For a trial period of a year, they are to act as though they were married.
2. If she is able to scare him within that year, the engagement will be broken. 

*Note: A three part short story.*

The Kuchisake-onna, Miroku-san is the bride-to-be of high schooler Kōichi Sano to regain her powers?! And for some reason, Kōichi is totally on board with the engagement… The exciting life of the Kuchisake-onna at the mercy of her high schooler fiancé starts now! 

(Source: MANGA Plus) 

*Note: Includes 10 extra chapters.*