This character only appears in the El-Hazard Radio program, which takes place
after the first OVA series.
For all extents and purposes, Ifurina is the TV version of Ifurita, but in the OVA timeline. She is the same ditzy and overly cute version of Ifurita that fans of the TV series love.
So how did she show up here? It's not very hard to explain, as she does it herself when the others meet her.
Ifurina is a demon god created during the great wars, or to be more specific, one of a series of Ifurina models constructed. Remember how in OVA 2, it is revealed that there were multiple Ifuiritas? It turns out that the Ifurita series was TOO destructive in power and hard to control. The ancient weapon makers decided to produce a less powerful and more docile demon god, and thus the Ifurina series was created.
This particular Ifurina unit was found by an innkeeper and decided to keep her around to help him run his restaurant. Unfortunately, time passes and the innkeeper dies. Ifurina, now masterless, still keeps to her duty and maintains the inn, mostly alone in the mountains.
When Fujisawa, the first customer in several years enter the establishment, Ifurina serves him liquor and does her duty well. She even chases down Fujisawa when he tries to weasel out of paying his bar-tab.
When the others discover Fujisawa, they also discover Ifurina. She displays several of the same traits as Ifurita. She even has her own key staff, which she rides like a witch rides a broom.
However, Jinnai also hears rumor of a new demon god and goes to investigate. While everyone's attention is occupied with Myuun and Salli, Jinnai takes advantage and becomes Ifurina's new master. From that point on, she mainly plays the role of his comic sidekick, much like she did in the TV series.
Ifurina doesn't garner much of the focus of the radio drama in the second half, she mainly briefly appears as a distraction to allow the others to escape Salli (and the only reason Jinnai even showed up was to brag about his new weapon, which she unfortunately proves to be as inept as her TV counterpart.) I think the main reason why she was renamed and brought into the story was because she had proved to be so popular in the TV series (and indeed, Yuri Amano's performance is top notch) that they created a plot so she could finally be brought into the OVA universe.
However, when production went into the second OVA series, while several ideas and scenes from the radio dramas were recycled. Ifurina, Salli, and Myuun ended up on the scrap heap and instead there is a semi-similar plot played out by a demon god named Kalia and a clone of Ifurita, thus preserving the ideal that there were multiple versions of the same demon gods. Who knows, if they ever do a third OVA or another TV series based on the OVA timeline, maybe Ifurina will be resurrected once again.