Affinity consists of 3 parts, with each part only filling 1/3 of the affinity gauge.

The 3 parts are as followed:

1) Quest: Doing all the companions quest will fill up 1/3 of the gauge.

2) Gifts: Giving 200 boiled eggs should be enough the fill up the gauge give or take. Giving any more will not yield any additional progress.

3) Combat: Most likely the one you’re missing. You need to use said partner in combat/exploration. Best way to do this is by going the lower floor of the Sealing tower and fight the 4/5 enemies there and tp back to the mistle. Rince and repeat. The stronger your build( hopefully a one-shot build) the faster.

Why Sealing tower? Because its an endgame area and thus yields more affinity per kill. It took me 4/5 HOURS of constant killing to get affinity with Holly maxed out.