Whether referenced in conversation or a character engaging with the cast, these are all the characters returning from Negima. Keep in mind, there are huge manga spoilers.
This page goes over ties in the UQ Holder timeline.
Yukihime/Evangeline A.K. McDowell
Once the great and powerful Magister Magi who saved the world. At the beginning of UQ Holder, it was revealed that Negi had died and the main lead, Touta was his grandson. Later, Mana revealed someone had signed up for the Mahora Martial Arts Tournament using the name "Negi Springfield". Cutlass reveals to Touta that he is actually a clone, which is has been confirmed he is from both Negi and Asuna. Negi and company show up at the Mahora Festival, possessed by the Lifemaker and attack Team UQ Holder.
Negi would make officials pactio contracts with Yue and Nodoka. We learn he, along with a team of his friends and companions, would go against Ialda, but they would lose the battle. She would try to possess him, but Jack and Alberio would same him from suffering the same fate. leaving him to fight another day. He would return to Mahora in grief. After losing many of his friends in battle, he would rest and recover with Evangeline. During this time, the two would get closer, and he would later break her curse. He made a plan to weaken the Mage of Beginnings by progressively lowering the amount of suffering in the world and would start researching ways to pass on his genes, which would lead to the idea of clones.
Before the battle, he and Evangeline would kiss, but whether it progressed to something more is undetermined. Negi would defeat his father, who was possessed by Ialda, but due to the absence of Asuna, the Mage of Beginnings would later be able to possess Negi. Though he disappeared and tried to hold her off, he would succumb to her possession.
She first appeared in one of Negi's flashbacks. It was revealed Touta is a clone with half the DNA taken from her, granting him the use of Magic Cancel. Part of Asuna resided in Touta and when the possessed Negi and friends fought UQ Holder, she joined Negi in order to delay their plan.
The swordswoman has only officially appeared in flashbacks thus far, but Isana Konoe, a new character to the series, bears an identical resemblance to Setsuna.
Has only appeared in flashbacks so far, but Honoka Konoe, a new 12 year old character, has an identical appearance to Konoka. In addition, another unnamed character has appeared and is working with Yukihime sometime before the start of UQ Holder. She looks strikingly similar to an adult Konoka, but whether it's her or not has yet to be determined.
When Fate first attacks the airport to seize Touta, one of his accomplices is Tskuyomi, a girl who bears the same face, name, and bloodlust as the crazed girl from Negima. She has achieved immortality by becoming a cyborg. She is also now apart of Ala Alba.
When he first appears, he's sided against UQ Holder, wanting to force Touta into helping him to save Negi. When UQ Holder beats him, they did manage to win some info from him. He and Yukihime have teamed up on several occasions when it came to their best interest. Fate has revealed his plan is to save Negi while Yukihime's is to ultimately kill him. He is now the leader of Ala Alba and Isana and Honoka Konoe, as well as Tskuyomi are apart of the group.
We learn he joined Negi in the battle with Ialda before UQ Holder starts, but they would lose the first battle and were unable to save Nagi or prevent her possessing Negi in the other. He, along with Yukihime, was looking into research about cloning, upon Negi's final request. He would later loan out the cloned experiments for money to fund his research. He and Yukihime would see his methods differently and would later part ways.
Now an old woman, she still offers Touta and UQ Holder help when she can. She first paid a trip to the inn with her granddaughter, Mizore, a love interest for Touta. Along with some of her other classmates, she explains a bit more about Negi at the Mahora School Festival.
Chachamaru would serve as Negi's secretary before UQ Holder qould take place until his disappearance. She has been seen accompanying Ayaka in the present in the series. She also helped inform Touta about more information of Negi during the Mahora Festival.
We learn that before UQ Holder would take place, Chisame would actively aid Negi in his tasks.She would later come to first battle against the Mage of Beginnings. As the Mage of Beginnings was about to land the winning blow on Negi, she would step in protect him, but would be killed in his stead.
Jack, now possessed by Ialda, is part of Negi's team. He retains the same characteristics as old before, but he has so qualms about killing people caught in the backfire of his attacks, as it's for the "better cause".
He would join Negi and the others in the first fight with Ialda, but he, along with several others, would be caught in her trap and would fall into her possession. He, along with Alberio, would be able to save Negi from the same fate, so he would be able to fight another day.
Part of Negi's current team. He's also possessed by Ialda. He showed Touta a piece of Negi's past as a ten year old and explains a bit of the situation to him.
He would join Negi and the others in the first fight with Ialda, but he, along with several others, would be caught in her trap and would fall into her possession. He, along with Jack, would be able to save Negi from the same fate, so he would be able to fight another day.
Ialda, The Mage of Beginnings
She was the final boss of Negima who possessed Nagi until defeated by Negi and Asuna. In UQ Holder, it is revealed she is responsible for turning Evangeline into a vampire. She controls Negi along with his companions, Yue, Nodoka, Alberio, and Jack. Through Negi, she interrupts the Mahora Festival and manages to trap Touta in her Phantasmagoria. She toys with Evangeline and attacks Touta, only to be saved by Asuna. Controlling Negi and his friends, they face off against UQ Holder in an attempt to acquire Touta for their end goal, but Asuna manages to save them and prolong Ialda's plan.
Having worked with Negi over the years, Yue's appearance in UQ Holder shows she still works with her former teacher, now under the influence of the Mage of Beginnings. She and Nodoka are a fearsome duo while attacking the UQ Holder team.
In a flashback, we see that she and Nodoka would request to be Negi's official partners after graduating 3-A. He would agree and she would fall into the possession of the Mage of Beginnings during the first battle with several others.
Like Yue, she is still with Negi, but her once timid demeanor is gone and is now a fearsome opponent. She works hand-in-hand with Yue, using her mind-reading ability to detect all enemey's moves. She also now sports white wings, though whether this is her outfit's feature is unknown.
In a flashback, we see that she and Yue would request to be Negi's official partners after graduating 3-A. He would agree and she would fall into the possession of the Mage of Beginnings during the first battle with several others.
She is first seen at Mahora Academy, challenging Touta to a fight to test his strength. She reveals she is acting headmaster of the school, filling in for a friend, and encourages Touta to sign up for the Martial Arts Tournament. She also informs him that a person with his grandfather's name signed up as a contestant. She reappears at the school during the school festival with some of her other classmates, talking to Touta about his grandfather and revealing that he was spotted. She also later helps save him from Fate.
She appears when a group of former students talk to Touta about his grandfather. She explains to him exactly what the Mage of Beginnings is doing to Negi and later saves him from Fate. She plays a vital part helping him plan his options to defeat Ialda.
She is first seen with a portion of her former classmates during the Mahora Festival, telling Touta about their old teacher.
While she has not directly played a part in the story, we learn that Karin had once been helped by Misora in the past. We also see her descendant, Mikan, working in the slums, also a nun.
She, along with Fate and the others, would go against the Mage of Beginnings in the first battle. In an attempt to prevent Negi's possession, she would try to stop time before she could possess him. However, her artifact was nulled and she was promptly killed by Ialda.
When Touta and Kirie first explore Nagi's old workshop, they run into an older gentleman, one skilled in the art of panties. He ends up stealing Kirie's and the other UQ Holder girl's underwear before revealing his true form. He turns into an ermine and lets the members know he was once the partner and advisor to Negi. When in that form, he slips into a more relaxed speech. He has been at the workshop watching over the lifeless Chacha-Zero for some time.