Phoebe Cuckoo and her sisters have left the Jean Grey Institute and have joined up with Cyclops' team of mutant revolutionaries and his school, as well.
During the events of Riot at Xavier's, Sophie convinces Mindee and the others to take Kick and use Cerebra to stop Quentin. Sophie leads by entering the machine and taking Kick while the others help her.
After they destroy Quentin, Emma reveals that Sophie had died in the machine. The four living sisters get revenge against Emma who they blame for Sophie's death, by alerting Jean to Scott's affair. Esme later shoots Emma in the imperfection in her diamond form and shatters. When it is revealled that Esme is the attempted murderer, it is too late as the institute is destroyed; and Esme has joined Xorneto. This backfired when Esme is killed by Xorneto, leaving the Cuckoos the Three-in-One.
After the deaths of her sisters, Sophie and Esme, Phoebe became the "de facto" leader of the other girls.
She normally voices their decisions, if not actually making the decisions. In Phoenix Warsong she wanted to know what she and her sisters could do with the Phoenix and wasn't scared of the power .
The girls later decided to help both Elixir and David. They gave elixir all the medical knowledge of Beast and gave back everything David had ever had learned using his powers.
When Belasco came to the school looking for Magik, the three were put into special helmets to prevent the use of their telepathy. When the helmets were removed the three were crucial in the defeat of Belasco.
The girls helped find Hope, before Cerebra was destroyed.
The girls helped with coordinating during Secret invasion, but when the thought wall was implemented, they were all but useless. Emma died when she tried to break the wall, and the girls were given the job of keeping her alive until she returned.
Quentin once again came back and was going to destroy the X-Men, but was stopped when the Cuckoos were alerted to his presence by Martha. They told him that they had become much more powerful no that their was only the three of them and the easily beat the omega mutant.
Irma was used to infiltrate the mutant prison that Norman created to combat the mutant presence in San Fransisco. she was captured which allowed the X-Men to infiltrate the base and save Xavier and Beast among others.
During the Predator-x battle on the island, the girls lost the Phoenix force that inhabited their hearts.
Powers & Abilities
Phoebe is a powerful and bright mutant telepath able to merge her mind with her sisters into a psychic gestalt. She can also transmute her biology into a diamond-like indestructible form.
Phoebe also possesses a piece of the Phoenix Force, but she (in addition to her sisters) has shunted off her access to the Phoenix.