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This one is heavily RNG based. It helps if you've not got the Highland expansion activated, only the characters.

Play as the Sprite character. Choose an Evil aligned character as your opponent - it can help to make it a "Local Player" you control, so the evil character doesn't accidentally take the Pixie with them.

Now you just need to run around until you encounter both the Pixie (Follower card) and the Fairy (Stranger card). The evil character won't be able to encounter the Fairy, but if the Sprite character does so without having the Pixie make sure to choose to not encounter it! Then, when the Sprite eventually picks up the Pixie as a follower, either return to the Fairy to encounter it, or run around until you do so for the first time. The achievement will unlock.