In the First Book, The Stone. Galen spikes the potion to get through the fire after everyone had left, knowing Quirrel would come to the right conclusion to the riddle and drink it, but not knowing its been spiked and thus useless. He burns.
In the Tournament Rin, Ilya, and Takara are all chosen to compete. All of whom have a crush of Shirou. Takara quickly realizes that there will be a catfight over him in the water.
When Rin uses the Kaleidostick it transforms her to her original state, who promptly kisses Shirou.
Shirou's first plan for dealing with Galen's romantic angst, dose him and Hermione with love potion and shove them in a broom closet with Takara for spice.