Rhodri can be found if you decide to try and uncover the truth about Lady Grey and her sister, Amanda Grey, during the quest Investigating the Mayor. He will be in Barrow Fields, near the Demon Door. When asked about Amanda's ghost, he tells the Hero of Oakvale to go to the carriage house outside Grey House and flash his lantern three times- this was his signal to Amanda to go to the cellar to meet him. However, according to what he tells the Hero, one of the times he went she didn't show up, and the cellar door was locked, which coincides with the time Lady Grey locked Amanda in the cellar to die.
If you decide to marry Lady Grey at the end of the quest and then return to Rhodri, he will make comments such as "Why must you and your awful wife keep tormenting me?!" and "So you married a serpent and turned into a snake!"
It is possible to kill Rhodri. The Hero must start in Bowerstone North after accepting the quest to dig up Lady Grey's past. After the Hero turns down Lady Grey's offer of marriage in the cellar, and then walks out of the cellar, through to Grey House, and then walks out of Grey House to Barrow Fields, the Hero must then talk to Rhodri, who will urge the hero to reveal Lady Grey's secret. The Hero must then teleport to Bowerstone South, and then walk to Bowerstone North, and take the job as Mayor. The Hero must then teleport back to Barrow Fields. The Hero should see Rhodri walking back towards the Barrow Fields traders, and may now kill him, as he will no longer be invincible.