Not much is known about Paca-tan's appearance, due to her not making appearances other than in Takeru's short imaginations. Only a few images of her exist, mostly with Takeru's speech bubbles obscuring parts of her. It can be inferred from the appearances of Alpacas and the few images of her, however, that she has white shoulder-length hair. She wears a parka (as an obvious pun to 'Paca') with 'VICUGNA PACOS' (scientific name of alpaca) written on the front and a miniskirt.
Paca-tan is an entirely imaginary Eto-musume made up by Takeru. After being explained by Nyaa-tan about the existence of other Eto-musumes for other animals, Takeru happily wonders if there exists an Eto-musume for alpacas, to which Nya-tan claims otherwise.
As the manga goes on, Takeru mentions her again and again, quite noticeably in Chapter 3 where he exclaims he will not forgive anyone who dares to hurt Paca-tan. This has led to suspicions that Takeru made his image of Takeru based on his personal preferences.
Paca-tan is an imaginary Eto-musume made up by Takeru, and hence it can be inferred that nobody knows Paca-tan like Takeru does. His declaration to protect Paca-tan implies heavily that Takeru is in love with his imaginary Eto-musume.
Although Nyaa-tan is quick to shoot down Takeru's question about the existence of an Eto-musume for alpacas, she seems to hold a grudge against alpacas nonetheless, as she claims in Chapter 3 that had Uri-tan been an Alpaca Eto-musume, Nyaa-tan would have quickly beaten her up (causing Takeru's outburst mentioned previously). Subsequent mentions of Paca-tan lead Nyaa-tan (and the others) to question who she actually is.
Paca-tan is a manga-only character, and only appears in a few panels.