Any non-pawn's affinity can be raised. There are a number of ways you can raise a person's affinity. The easiest way is to give them presents. Press to speak with them, then press to choose a present to give them. Certain items will raise certain people's affinity more. If you look at the details of an item it will say if there is a specific person who favours it.
There are a couple methods that allow you to max someone's affinity quickly.
The easiest one is to complete the "Arousing Suspicion" quest. This will instantly raise Aelinore's affinity to maximum.
Another way is to do the quest "Talent in Bloom". To do this speak to Quina at the Abbey. Quina is the girl who helps you at the very beginning of the game. To find the Abbey, go west through the forest from the ancient quarry's north entrance. Present her the flower she gives you and complete the quest. If you're having trouble finding the Abbey, refer to this map.
Once you've reached maximum affinity with someone, you will know because you can hear a ringing noise each time you speak to them. The achievement should unlock as soon as their affinity has reached max.