Ruling 2343
The surety must be of the age of legal responsibility (bāligh), sane (ʿāqil), and no one must have compelled him [to enter into the kafālah agreement]. In addition, he must be able to make the person for whom he is the surety appear, and he must not be foolish with finances (safīh).[1] Furthermore, he must not have been proclaimed bankrupt (mufallas) in the event that making the debtor appear requires him to have disposal over his property.
-Ayatullah Sistani, Practical Laws of Islam, Surety for the Appearance of a Debtor (Kafālah)
1
Ruling 2091 provides further clarification of this term: it refers to someone who spends his wealth in futile ways.