I'm Friends with the Female Side Character

She transmigrates into a book as the original heroine, a rich beauty who gets reduced to cannon fodder in a story about a supporting heroine. She doesn't want the increasingly successful supporting heroine to outshine her, but she can't take her down either. The other girl has a spiritual spring space and is even ridiculously strong.

What can she do?

Heroine: There's nothing for it. I'll just have to be friends with her.

*

Supporting heroine: What the heck... what's going on lately? Why does the heroine keep buying me bags, jewelry, and skincare products?

Supporting heroine: What the heck... a windfall! What the heck, the heroine gave me a card and told me to spend whatever I want... I have no way to repay her. Maybe I can give her some tea leaves boiled in spiritual spring water and then dried in the sun...

Supporting heroine: What the heck, is there something wrong with the heroine's boyfriend? He dumps her and tries to flirt with me instead? He's asking for a beating! Come on, scumbag, take a punch!

Supporting heroine: I want to follow and protect the heroine for the rest of my life.

In one sentence: The heroine and the supporting heroine become friends. The supporting heroine protects the heroine for life, and her very existence is the heroine's greatest golden finger.

[Old blurb from a few years ago, screenshot saved]

Theme: Even when people have different identities and loyalties, that doesn't mean they can't eventually become friends.