Beamspam McMuppet — Atenborough, the Gonkish weapons officer of the DaiGurren, who loves his work a little too much.
Also known as Button Smash McMuppet.
Captain Garlock — the captain in the prologue to the first episode, who resembles Captain Harlock. He is believed to be an alternate version of, and should not be confused with, the much less arrogant Post-Timeskip Simon, who is referred to as Captain Starlock.
Cotton Hill — Makken of the Team Dai-Gurren, who resembles Hank Hill's father.
Doombitch — Yoko, who gave a Last Kiss to two characters, Kamina and Kittan, who then died of Retirony.
In her defense, Kittan was the one who kissed Yoko, and that was AFTER he decided he was going on a suicide mission to give Team Dai-Gurren the chance they needed to win.
Professer Midrift — Otherwise known as Leite. Named as such due to nearly every scene she's featured in displaying her nerdy abs.
Bruce Ironstaunch — An otherwise unnamed background character who only appears in one scene in the series, making a supportive, strong-armed gesture to Rossiu during the speech where he gives news of Simon's death sentence and word of an escape plan to save the city's population from their planet's moon, which is set to collide with the planet. In Western culture, however, Bruce's gesture translates roughly to "fuck you," and as such several fan artworks of Bruce have been created by Western viewers, depicting Bruce beating on Rossiu or just being manly in general.
The same memetic Fanon holds he's the one who threw that anonymous rock at Rossiu. But, whoever it was, there is at least one fan club dedicated to them.
Lulussiu and Spinsimon: Rossiu and Simon, respectively after the Time Skip, because the same English voice actors voice bothpairs, and because of both pairs' similar actions and relationships. Also note the oddness that each voices the generally like main character in one series, and The Scrappy in the other.
Namely, Rossiu's penchant for Xanatos Gambits and Simon's Uhhh. Spinning.
Within the series, Kamina's voice actor also voices Makken (Leyte's husband and the pilot of the Mo-Shogun). He's credited as "Reo Kaminaga", which is an anagram of the Japanese words for "I am Kamina."
While we're at that, Kiyoh (the eldest of Kittan's sisters) is Kallen. It's like they walked from one room to the next when it came to auditions. In fact, they've always been voiced by the same actors, they just don't get into the familiar voices until the timeskip.
Dub!Nia is voiced by Hynden Walch (Starfire from Teen Titans) and boy does she ever put it to good use. The world figures for diabetes were painted red when she described Adine as a "Pissing off person".
What Could Have Been: A bit of a subversion. In the original tv series, there just wasn't enough time or money for the animation team to put in everything they wanted, which is why so many of the cast were killed off. Thankfully, the series was successful enough to warrant film adaptations, and the second film includes all of the content they originally wanted to add in.