who is the guarantor

Here is the 2nd part of the whitepaper, which gives a high level in-depth view of the trustless algorithm we use. Please refer to the part 1 if you need to understand some terms. Part 1 is here:
https://bitcointalk.org/index.php?topic=618552.msg8272890#msg8272890

==
The following diagram shows a high level description of the trustless system algorithm. It shows the “normal” case where everything goes as expected.




The next diagram shows the case where, after step 6, the Sender is not satisfied with the Mixer’s txid. This could happen if the Sender cannot verify Mixer’s transaction, or Mixer did not send enough funds to the destination. In which case Sender asks Guarantor to do the arbitration. The new scenario are marked in brown lines and explained in the diagram.



There are other possible scenarios, that we will describe in the next parts, where we will show details of the algorithm and steps. But from the above two cases you see why multisig is tightly linked with trustless system and how it creates a bonding among all parties where they have to follow the anonymous transfer rules.