The Alwars (also spelled Azhwars) are a group of 12 ancient Vaishnava saints who lived in Tamil Nadu and are famous for their poetry in praise of Vishnu. The Alwars are crucially important figures in the development of Vaishnavism; it is the principles and beliefs embodied in the Alwars' poems that ultimately gave rise to the Sri Vaishnava sect (of which I'm a member). Now one of the Alwars was known as Thondaradippodi Alwar, who is considered an incarnation of Vishnu's Vanamala garland as I discuss here. He gets the name Thondaradippodi, which means "the dust of devotees' feet", because he was so humble that he wore the dust of Vaishnavas' feet on his forehead.

In any case, in this poem from Thondaradippodi Alwar's collection of poems Thirumalai, he says this in praise of the Ranganatha statue in Sri Rangam (which I discuss here and here):

But my question is about the figure Kshatrabandhu alluded in the verse. Kshatrabandhu is also mentioned in Thirukkolur Penpillai Rahasyam, which as I discuss in this question is a list of 81 statements told to the Sri Vaishnava Acharya Ramanujacharya by little girl from the town of Thirukkolur. The ninth statement is "mUnRezhuththuch sonnEnO kshathrabandhuvaip pOlE", meaning "Am I like Kshatrabandu who said three letters?" Here's how this web page explains the meaning of this statement:

The Alwars (also spelled Azhwars) are a group of 12 ancient Vaishnava saints who lived in Tamil Nadu and are famous for their poetry in praise of Vishnu. The Alwars are crucially important figures in the development of Vaishnavism; it is the principles and beliefs embodied in the Alwars' poems that ultimately gave rise to the Sri Vaishnava sect (of which I'm a member). Now one of the Alwars was known as Thondaradippodi Alwar, who is considered an incarnation of Vishnu's Vanamala garland as I discuss here. He gets the name Thondaradippodi, which means "the dust of devotees' feet", because he was so humble that he wore the dust of Vaishnavas' feet on his forehead.

In any case, in this poem from Thondaradippodi Alwar's collection of poems Thirumalai, he says this in praise of the Ranganatha statue in Sri Rangam (which I discuss here and here):

But my question is about the figure Kshatrabandhu alluded in the verse. Kshatrabandhu is also mentioned in Thirukkolur Penpillai Rahasyam, which as I discuss in this question is a list of 81 statements told to the Sri Vaishnava Acharya Ramanujacharya by little girl from the town of Thirukkolur. The ninth statement is "mUnRezhuththuch sonnEnO kshathrabandhuvaip pOlE", meaning "Am I like Kshatrabandu who said three letters?" Here's how this web page explains the meaning of this statement:

EDIT: I just found another reference to Kshatrabandhu in this excerpt from the Rahasyatraya Sara, a work by the Sri Vaishnava Acharya Vedanta Desikan:

It is said in the Smritis: "Kshatrabandhu, who was the worst of sinners, and Pundarika, the virtuous - both of them obtained moksha or release from bondage by virtue of their having acharyas." It is thus declared that in the case of every one, the only means of securing moksha is to have an acharya.

But I'm not sure what Smritis he's referring to.