Some TMNT stuff really isn't for little kids.
At some point prior to the series, April considered dropping out of college in order to help care for her father after his debilitating stroke. Instead, she gained an internship with Stock Gen Research Inc., believing that the bioengineering company only intended to supply food to starving people across the world. It was during this internship that she first encountered Splinter and the Turtles in their pre-mutated forms; she named the latter after artists from her Renaissance History class.
One night at the lab, April was surprised by the arrival of two ninjas who stole chemical samples and the baby turtles. Splinter triggered the fire alarm, allowing April to fend off the ninjas before they could kill her. This incident proved to be extremely traumatic for April, even after the passage of several months.
She later offered tutoring in exchange for self-defense lessons, and encountered Casey Jones. As he was teaching her how to defend herself, she told him of the StockGen break-in, including Splinter's role. Recognizing the name, Casey brought her to the Turtles' lair, where she fainted upon seeing four human-sized Turtles. Upon awakening, she realized that the Turtles were the same test subjects that she had bonded with at StockGen.
Wishing to help them, she assisted the Turtles in breaking into StockGen in search of Splinter, which they eventually succeeded in doing. As their home had been destroyed, she also briefly allowed them the use of the Second Time Around shop and its apartment.
Her help to the Turtles did not end with housing; she later used her position at StockGen in order to spy on her coworkers and covertly steal a Turtle Tracker, which nearly caused her to be captured by one of Krang's Stone Soldiers. It also led to further complications in her life, such as attacks from Slash, the Neutrino soldiers and the Foot Clan.
However, the greatest complication came when Shredder captured Leonardo and stabbed Casey Jones. She and Casey began a relationship during his recovery. When the Turtles managed to extricate Leonardo from Shredder's clutches, April used Harold Lillja's stealth technology and antigravity gauntlet to fend off Rocksteady and Bebop.
In the aftermath of this incident, she fled with the Turtles to her family's farm in Northampton, under the pretense of bringing her boyfriend to meet her parents. However, an attack by the Foot Clan's assassins forced her to head back to New York with the Turtles, but not before giving a vial of ooze to her mother, allowing the effects of her father's stroke to be reversed.
After returning, she resumed her status as a full-time college student, where she discovered that Foot Clan expert Dr. Miller was lurking on her campus. From him, she learned of the existence of a Pantheon of powerful immortals in an ancient scroll he obtained from the Foot Clan. To find an accompanying scroll, April and Casey journeyed to the Southwest, where they encountered the Rat King and his more benevolent sister Aka. Aka developed a fondness for April, believing that she would play an important part in the coming war.
This experience led to a breakdown in April's relationship with Casey, as the Rat King had preyed on Casey's fears of his own weakness. However, April's encounter with Aka had more immediate effects, as she taught April a spell that nullified Kitsune's powers, allowing April to stop her from killing Splinter and turning the Turtles against one another. She continued her research on the Pantheon in the Foot Clan library,
She took some time out of her studies to confront Baxter Stockman, warning him that while he may have barely survived his dealings with Krang and Shredder, there were new threats that were not so easily dealt with. While Stockman was initially hostile to her offer of a truce, he was forced to ally himself with her when the Earth Protection Force surrounded the T.C.R.I. facility. She managed to save her friends by leaping onto the roof of a nearby building, suffering a mild injury in the process, and brokered a deal with Null to save the Turtles. This incident led Stockman to agree to work with April.
April was one of the guests at the Turtles' Christmas party, which also allowed to her interact with Casey once again.
During the Triceraton invasion, April was forced to remain in Stockman's lab, where she assisted the Turtles by filling them in on what was going on.