Here is the full list of all 19 Lessons with Chii-chan achievements.
Had your first lesson with Chii-chan.
Enjoyed Chii-chan's special chocolate pudding.
Pretended to be Chii-chan's brother.
Narrowly survived a life-or-death battle with Chii-chan Number 2.
Got a quick lesson in at the library.
Celebrated your birthday with Chii-chan.
Went to the park to take refuge from the heat.
Cooled off after a long, hot day.
Went to a fireworks festival with Chii-chan.
Warmed up under the kotatsu with Chii-chan.
Looks like the teacher has become the student!
I'll give you one guess.
Participated in a showdown with Chii-chan.
Warmed up thanks to a hot, steamy bath.
Had a therapeutic heart-to-heart with Chii-chan.
Beat the game and saw the ending.
Experimented with a new studying technique.
Went to the countryside for a summer getaway.
Had an awesome summer with Chii-chan by your side.