In the Fragments tab, you'll find a few rare types of soul fragments. Some of these may be worthwhile to you, especially early on, but weigh their cost vs the cost of relics you wish to acquire.
Are you picking up that relics are where it's at yet?
With the recent update in gauntlet characters, these fragments became the backup option to get the "gauntlet toons". At 50 fragments for 2000 IC and only one buy per week, this is both long and expensive since you need 500 frags for each of those toons.
Is at the moment of writing the most coveted one. Excellent offense and versatility make her the best choice.
Necropolis, The Ancient Mariner
Are sub-par choices, collectors will want them, but when you finally have them, the aren't better than the other 5* toons you've been getting elsewhere.
A nice but slow way to get the eternal toons (see frontier store) and when you have them all, regular 5* fodder for the sacrifice events.