Because of her severe debt, she made a contract with the World to become a Heroic Spirit. Having decided that there were too many Grail Wars occurring across the different dimensions, and because she was a relatively new Heroic Spirit, the World tasked TOHSAKA with destroying the various Holy Grails, reducing her debt slightly for each one eliminated.
Initially, TOHSAKA, whilst vaguely recognising both Shirou and herself, is exceedingly flippant towards her role, dramatically reciting her history and declaring herself as a Wandering Witch. She shows no concern for the lives that she ends, as the backlash from destroying the grail will evidently wipe Fuyuki City from the face of the earth.
After being defeated however, she is reminded of her past, and decides to turn over a new leaf. While she is no longer focused on the destruction of the grail, her attention on money does not change, being equally obsessed and equally frivilous with it as before.
TOHSAKA has a tendency to mix English in with Japanese in her speech patterns.
Confronting Shirou, TOHSAKA remarks that he is vaguely familiar to her before explaining her history and her reasons for becoming a Heroic Spirit. When she mentions potentially destroying the town along with the grail, Shirou resolves to fight her.
After defeating her, TOHSAKA realises that it was a mistake to make a contract with the world, and instead decides that it is much more sensible to instead take it over and hire out heroic spirits to other people for a fee.
Wrangling Shirou into helping her advertise her new business venture, the pair take to the streets to hand out fliers for the TOHSAKA Heroic Spirit Service, with the younger protagonist deciding it was better not to point out how slimy her plan was.
Beginning much the same as Shirou's encounter, Rin's motivation for fighting her older self is just to remove her before anyone sees what she could potentially become.
After the battle, Rin finds out about TOHSAKA's debt and the amount of interest she was paying, and after some quick calculation, discovers that TOHSAKA had already paid off the debt to the point where she was actually due to be reimbursed for over paying.
Elated at the news, TOHSAKA takes Rin out shopping and accidentally ends up accumulating another debt. Forced to once again hunt down grails, Rin is also contracted in as Elementary School Heroic Spirit RIN-san, much to her dismay.
As a master in Capsule Servant, TOHSAKA is capable of multiple small scale summonings of pseudo-Heroic Spirits, or Capsule Summoning. Her Master Skill is Shin Tohsaka Money Is Power System (真・トーサカM.I.P.S, Shin Tōsaka Manē Izu Pawā Shisutemu?).
She also possesses a “IOU (Self Geass Scroll)”, which is seemingly the magical contract that keeps her in constant debt.
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Capsule Servant
- ↑ Capsule Servant, Shirou Route
- ↑ 3.0 3.1 Capsule Servant, Rin Route