Henry was one of the better received characters introduced in Season 8 for being a Nice Guy and is generally considered a good match for Paige.
Chris. His appearance on the show sparked massive fan love in Fanfiction.net. His character can be found under TWO character filters, Chris H. and for some reason, Phoebe's boss Elise R. (this probably because his character became popular before FF.net gave him his own place in the list). Apart from that, the Chris H. character filter generates more pages than anyone else on the series. Though this can be explained by the lack of filter in the earlier days of Charmed, it's pretty impressive for a character who was only a regular for one season. Nowadays, Chris fics make up most of the updates and new stories in the fandom.
Kyra the Seer. Only appears in three episodes but is fondly remembered thanks to being witty, entertaining and surprisingly deep. Also she's played by Charisma Carpenter - making this the most high profile of the Buffyverse actor crossovers.
Also Melinda Warren. You'd be surprised that the character only actually appears in one episode - in the first season at that. Due to being the start of the Charmed line and possessing all three powers, as well as her interesting backstory, she's a frequent choice for fanfiction. Ditto for Eva, who also was only in one episode - yet enjoys great fan press as The Mentor to Melinda.
It was recorded which cast member received the most fan mail and it wasn't any of the leads - it was Finola Hughes. Which would make Patty an Ensemble Dark Horse too.
The half-manticore from the season six episode "Little Monsters" is ridiculously popular, considering that he was a oneshot character, a baby, and didn't even have a name. He often is depicted as the next generation of Halliwell's best friend.
Despite dying in her debut episode, Bianca from "Chris-Crossed" is very popular, due to being a badass, her relationship with fellow fan favorite Chris, and the interesting possibilities that she represented. Some were disappointed that we never found out about her life in the changed future or if she and Chris got together again.
The amount of mentions, love and fanfictions Mark Chao gets is astonishing, considering he was a one-shot character who appeared in one of the earliest episodes of Season 1 and was dead (albeit a ghost) upon introduction. When John Cho appeared in on the House of Halliwell podcast he mentioned that he still gets people asking him about playing Mark.
Drake de Mon remains as one of Phoebe's most popular love interests, despite his short tenure on the show, due to his charming personality, fun episodes, role in helping Phoebe regain her faith in love, and interesting backstory. Many fans wished that the sisters could have saved him, even if he didn't end up as Phoebe's endgame love interest.
While acknowledging that their relationship was rushed for the finale and Coop is often a victim in Die for Our Ship to Phoebe/Cole fans, his approach to love and personality has made him a favorite among some fans, especially because his relationship with Phoebe being rushed aside, he does appear to be a good match for her.
The Angel of Death is only in three episodes, but is very memorable for Simon Templeman's performance, and quite well-liked for being a True Neutral and Noble Demon.
True Neutral and Noble Demon.