It's a long manga with a female high school MC. She unexpectedly made a pact with an ancient magical item (I forgot what it was) to learn how to use its power to protect herself. Her love interest is a older man (I think he's her calligraphy sensei?) with a mysterious past who was supposed to be the head of a powerful clan but got banished/isolated from the clan (so his younger half-sister is the head instead). He was trying to collect several powerful items to defeat some evil demon who killed his wife and left his sister crippled many years ago. Other characters include a hot tempered high school boy (love rival) who MC constantly argued, and a cold friend/assistant to the male lead (who was actually the clan's spy?). The MC later discovered her complicated past which involved the clan and why her crush resembled the little boy she thought had saved her life when she was younger.

I've been trying to search for the name with tags and genres but no result. Any help would be appreciated.