The "Titan encountered by Ilse Langnar on the 34th expedition" was a former member of a cult that followed Lady Ymir in Marley before she was turned into a Pure Titan when the group was discovered by the Public Security Authorities.
Appearance: This woman appeared to be a young adult with short blonde hair and bright eyes. Her Titan form was about 6 meters tall, had a very large head for its body size, a mouth with constantly exposed teeth, and short blond hair similar to its human form. It was also seen walking on all fours at certain times.
History: As a member of the cult of Ymir Fritz, this woman worshiped a girl by the name of Ymir who was brought into the cult by its leader. However, when the cult was discovered by the Public Security Authorities of Marley, the woman was taken through the streets with the rest of the cultists and Ymir herself until they were sent overseas to the borderline of Paradis Island, where they were all turned into Pure Titans.
Story: During the 34th expedition beyond the Walls as Ilse Langnar flees towards human territory, this Titan finds her and corners her against a tree. Rather than eating her, it instead stares at her until, to her amazement, it speaks. The talking Titan speaks of her as "subject of Ymir" and afterward addresses her as "Lady Ymir" before greeting her with a formal bow.
Abilities: This Titan was able to speak words with meaning to Ilse Langnar. It also displayed emotions which regular Titans lack, such as reverence, anguish, and perhaps fear.
Source: Talking Titan, available under CC BY-SA 3.0.