A Whispering Death trained by Alvin the Treacherous (but don't hold that against Groundsplitter!). Hiccup was almost able to tame its wild ways... almost.
— Dragons: Rise of Berk
Groundsplitter looks very similar to all the other Whispering Deaths that have been seen thus far in the franchise.
In her debut, Groundsplitter has the typical personality of her kind; highly aggressive, violent, and dangerous. With these character traits, she seems to be a perfect match for Alvin, which allows her to bond with him.
However, she does have a maternal side as she knew the Screaming Death was her offspring despite not being present when it hatched and was very happy to meet and be with her offspring.
She also seems to trust Hiccup and Toothless as they reunited her with her family and tried to help her when she was captured by Dagur, as she rescues them when the Screaming Death did not recognize them.
Some time Alvin finally got his own dragon, the Whispering Death, Groundsplitter. Alvin had captured and caged her alongside other dragons on Outcast Island. With the help of Mildew, Alvin managed to trick Hiccup into revealing his dragon training secrets. Afterward, Alvin tamed her along with Mildew in the Outcast Training Arena.
The Outcasts placed crates of Groundsplitter's eggs under Berk in an attempt to destroy the village. The offsprings, one being a Screaming Death, attacked the village in Tunnel Vision, but were chased off by the Dragon Riders.
Groundsplitter appeared with her eggs and it is revealed that the Outcasts had more Whispering Deaths, one being fed by Mildew, possibly Groundsplitter's mate, and the she was the Screaming Death's mother. Hiccup realizes that the Screaming Death was destroying islands trying to find her, thus Alvin releases her and the other Whispering Deaths during the fight with the Berserkers.
Fishlegs leads the Screaming Death to the island, which the mother immediately recognizes as her child. Dagur and some of his men capture her and threatens to kill her unless Hiccup hands over Toothless. Thankfully, Snotlout and Hookfang show up and free Groundsplitter. She and her offsprings reunite with the Screaming Death and the family leaves Outcast Island together.
It is revealed that Groundsplitter had been living on an island with the Screaming Death, and possibly her other children. While the Screaming Death did not remember Hiccup and Snotlout, she did. She defended them when they came to the island from her large offspring, allowing them to take some of the Screaming Death's shed scale to make armor against the Dragon Hunters' arrows.
Alpha: Being the mother of her Family, Groundsplitter is shown to have complete control over them, including the Screaming Death.
Intelligence: She has shown to be able to remember her saviors from few years ago, as she was able to protect them from the Screaming Death.
Groundsplitter's name was first mentioned in School of Dragons as one of the Flight Club courses. It was not mentioned who trained it.
Groundsplitter is the first dragon trained by Alvin & The Outcasts.
Like all dragons trained by the Outcasts, Groundsplitter was captured and caged to be trained.
According to Rise of Berk, Hiccup was close to training this dragon. When that happened is unknown, considering that the only Whispering Death Hiccup tried to train in the TV series was Toothless' Nemesis.
However, its possible that Groundsplitter is indeed the same dragon and was captured by Alvin some time off-screen, though this has yet to be confirmed.
Groundsplitter was speculated to be the Screaming Death's father.
However, according to the Rise of Berk developers, Groundsplitter is the Screaming Death's mother, which means its female and the same Whispering Death that saved Hiccup from the Screaming Death in Race to the Edge.