His surname "Morichika" means "near the forest", which is the location of Kourindou. His first name means "helps the long rain", and it possibly has something to do with Marisa Kirisame since her last name, "Kirisame", means drizzle.
Rinnosuke seems to be the only depicted humanoid male that has been introduced in the Touhou series so far.
Rinnosuke apparently doesn't have any fighting ability like Reimu or Marisa do, which makes him an ideal character to describe typical daily life in Gensokyo.
Rinnosuke worked as a shop assistant in the Kirisame house some time ago, so he and Marisa Kirisame have known each other since then.
Rinnosuke was the one who gave Marisa the Mini-Hakkero. He stated that it is his own invention and that it merged the magic in Gensokyo and the technology from the outside world (to which he refers as an outside form of magic that needs no spiritual power). He has also repaired and upgraded Mini-Hakkero at least once.
Only Marisa is known to call him by his nickname, "Kourin". This implies that they've known each other for a long time. However, "Kourin" is a standard nickname for him amongst fans.
Rinnosuke possesses a kerosene fan heater, which is one of his items from the outer world. The heater's fuel is supplied by Yukari Yakumo.
Rinnosuke and Yukari Yakumo are often credited as the two people who have some idea regarding the outside world, with some basic understanding of technology. Other characters may also know about the outside world, but there is no direct description by ZUN so far, with the recent exceptions of Sanae Kochiya, Kanako Yasaka, and Suwako Moriya, who are all from the outside world.
In Marisa's ending in IaMP, she mentions showing Suika to Kourin.
Rinnosuke may be the only literary character to be mentioned in a game dialogue (not by name) when Nazrin mentions obtaining the Jeweled Pagoda from "a second-hand goods shop".
He is also one of the few characters that doesn't wear any headgear type.