[@Rune_Alchemist]
Oh, absolutely! As a matter of fact, the better rapport you have with a summon, the stronger the link will be over time, which will make invoking the being easier.

Yes, [@Hammerman] is right... basically when you summon something, it becomes 'yours' unless the link breaks. 
[hr]
Also, everyone, I've been working on clarifying the level/tier system, since I felt it was a little foggy. Hopefully this bit if info helps you with your characters. Keep in mind we are sticking to Adept and Virtuoso (I mean...unless you really want a Tyro...no Masters, though, sry).

[hider=Levels explained here]
[u]Tyro[/u]
Students at this level are just beginning their studies of Ars Invocato. Tyros learn to concentrate their powers on searching the realms and opening gates.

[u]Adept, Tier 1 (A1)[/u]
These students have shown competency with Ars Invocato. They can summon basic beings such as small animals, lesser spirits, and simple tech. Their links are, at times, weak or easily severed.

[u]Adept, Tier 2 (A2)[/u]
Students in the second tier have proven to be more capable at invoking and can build stronger links with their summons. Though they have little trouble invoking beings of a slightly higher quality than A1 students, they still have a lot to learn.

[u]Virtuoso, Tier 1 (V1)[/u]
This level is reserved for students who have excelled in their study of Ars Invocato. Invoking higher beings is a breeze and nearly every summon is solid and harmonious. Students at this level begin to experiment with creating summons from their mind.

[u]Virtuoso, Tier 2 (V2)[/u]
These students are just shy of mastering Ars Invocato and taking their studies to the next level. They have extraordinary magical talent and no problem invoking powerful and complex beings or creating original summons from their thoughts. Some V2 students have shown capability for invoking multiple beings at once, though oftentimes, the links are not as strong.

[u]Master, Tier 1 (M1)[/u]
These students have taken Ars Invocato to the next level. Master students have zero problems invoking extremely dynamic beings with ease. Many Master level students can hold strong links with multiple beings while exerting little effort.

[u]Master, Tier 2 (M2)[/u]
M2 students are even more capable than their first tier counterparts. They are the elite of L'Mordryn and have shown flawless execution of Ars Invocato in every aspect.
[/hider]
[hr]
I've also adjusted the age range for characters to be 15yrs - 21yrs. I think it was a bit young before, but it's still gonna be young enough for a school environment.

I'm really glad you all are so into this! I'm liking what I'm seeing with the characters so far! I don't want to fully evaluate anything, though, until I feel I've got all info covered.