With Levi and Tenrou Island arc Gajeel got way more development than Juvia has gotten in total and worse Gajeel got way less panel time but way more development. Not only was he Makarov's spy on Raven Tail 7 years ago, we got an amazing back story. Plus as you know from reading the manga Gajeel is getting more plot coming his way too... and Im not referring to Chariot which airs in ~24hrs. So stating Gajeel as getting sidelined too makes Juvia's sidelining ok is a fallacy argumemt since their levels of plot sidelining are not equal, Gajeel being a Dragon Slayer and having an actual dynamic romance building instead of a static non romance truly disproves that entire argument you just made.
Agreed Gajeel gets not much plot when compaired to main characters but he gets way more than Juvia whom is trying to date the 4th main character and by that note should be getting more development than Gajeel (excluding DS stuff). However this isn't the case and in fact the opposite is true... Gajeel who's interested in an even more side character than he gets way more plot and development than the girl trying to date the main male supporting character.
Oh and they didn't get to me... but the passion of defeating them did , if that makes any sense
