Who - What - Why

One who is always behind a person to protect is Tatpurusha.
This Tatpurusha aspect is correlated To “A” in 'Omkara'
The form of Shiva that is in meditation and Blessing the world.
Tatpurusha is the Eastward incarnation of Maha Siva being of yellow complexion and of deluded or misled Purusha.

tat-purusha = "that-man" in the sense of "that person's man". (genitive)

tatpurusha (तत्पुरुष) compound is a dependent determinative compound
(a compound of two words in which the first is an attributive noun and the second a substantive)

There are many tatpurushas (one for each noun case, and a few others besides);
in a tatpurusha, one component is related to another.