You first meet Sebastian, a Slytherin student, during the main quest "Defence Against the Dark Arts Class".
As you progress through the story, Sebastian's quests pop up in your field guide. There are a total of 14 quests to complete for this achievement:
In the Shadow of the Undercroft (Main Quest)
In the Shadow of the Estate (Main Quest)
In the Shadow of the Bloodline (Side Quest)
In the Shadow of the Study (Side Quest)
In the Shadow of Discovery (Side Quest)
In the Shadow of Time (Side Quest)
In the Shadow of Distance (Side Quest)
In the Shadow of the Mine (Main Quest)
In the Shadow of the Mountain (Main Quest)
In the Shadow of Hope (Side Quest)
In the Shadow of the Relic (Side Quest)
In the Shadow of Fate (Side Quest)
The Final Repository (Main Quest)
In the Shadow of Friendship (Side Quest)
For most of his quests, you receive EXP. However, he presents you with options to learn several spells!