There are gods in the Rigveda who are thinly attested, and then there is Aja Ekapada. He has no hymn. He is not described. He performs no act, holds no attribute, receives no epithet, and is not addressed on his own anywhere in the collection. What he has is a handful of appearances in enumerations of gods, and in almost every one of them he stands next to Ahi Budhnya, so that each of the two is known mainly through the company of the other.
The classification usually given to him, an atmospheric deity, is not drawn from anything said about him. It is drawn from where he is placed in the lists, which is among the gods of the middle region, and from the assumption that a name paired with a serpent of the deep belongs to the same department. That is an inference from arrangement, and it is a reasonable one, but it should be recognised for what it is. Nothing in the text puts him in the sky rather than anywhere else.
What is genuinely visible is the mechanism that preserved him. Vedic enumeration is formulaic. Once a name enters a list it travels with the list, and the list is recited long after anybody remembers what the name was for. Aja Ekapada is what a formula looks like when its content has gone: a slot that continued to be filled correctly for three thousand years by people who could no longer say what they were filling it with. The lunar mansion scheme, which makes him the presiding deity of Purva Bhadrapada, is the same mechanism working one system later.