's illustrator, Kentaro Yabuki's wife was a total jerk (See the Creator Breakdown
entry on the work's page to read about some of the BS she pulled), so one of the main love interest, who was based off of her, had her role severely toned down by the writer. This was, of course, rather noticeable.
Not to mention, after the inevitable messy divorce caused the series to be cancelled, it was resurrected as To Love-Ru Darkness
and followed a very different direction, one that changed the main plot issue to something that doesn't necessarily require the character in question to share the main stage with the other two main links of the Love Dodecahedron
, thereby making it less noticeable.
So, in short, the person who one of the main characters was based off of turned out to be a jerk, so her role was significantly diminished, and disguised as well as they could manage.