The "Hange Zoe" of NieR Automata, as many fans refer to her, Jackass is a curious character. When 2B and 9S first meet her in episode 3 at the Resistance camp, she's ecstatic at the opportunity to have met some YoRHa androids, and immediately asks their permission to be taken apart for "research".
A comic relief character in an otherwise dark series is always welcome, and it's a bonus if they provide value to the plot. In Jackass's case, not only is she a key character to understanding the Resistance, but also a useful ally to 2B and 9S's mechanical adventure.
Who is Jackass?
Jackass is one of the members of the Resistance that the player meets early on in NieR: Automata, she primarily acts as a character that you can do side quests for. In game, she asks 2B and 9S to fight in a series of battles against lesser machines, so she can further collect data. After the quest is complete, she gets to work on a drug that will enhance combative abilities using the data she collected from observing the player's actions. She plays a relatively small part, but interacting with her gives players a broader view of the world that is NieR: Automata. In-between tasks, she shares her findings with the YoRHa androids; one of which is a reaction in androids' brains that simulates pleasure during battle.
Jackass frequently equates the thrill of battle to the human emotion of "love" and notes how without that circulatory reaction, they would've stopped fighting a long time ago. In NieR Automata Ver1.1a, she's introduced as a core part of the story, as opposed to another NPC the player comes across in their journey. Additionally, from the getgo, we already know she's an eccentric "scientist" and has a morbid fascination with taking things apart. Regardless, her character provides a good insight on the divide between the Resistance and YoRHa.
What's Her Role?
As stated prior, Jackass's role in the original game is small, and she serves primarily as exposition. But seeing how she was introduced in the anime, and how smaller parts of the NieR Automata lore made it into its production (ie. 9S's bag having a "mystery" item in it), she could serve a larger role in the grand scheme of things. So far, she helped 9S and 2B reach their destination in episode 3 and assisted them in battle. Parts of her initial game introduction were included as well; namely when Jackass is asked why she's standing so far from an entry point shortly before it explodes. In the anime, a similar interaction happens, but this time they're all in a truck. Instead of the explosion being a passive occurrence, it's a core event. With this, we can safely assume that more content from the game could be incorporated into future episodes, now that Jackass has a larger role.
Jackass primarily exists as an exposition character in the original game, though the knowledge she provides to the players could be an important plot point in the anime adaptation. And what specific knowledge might that be? Her thesis on love, pleasure, and violence. If you couldn't tell already, NieR Automata is 2B and 9S's story, and there's undoubtedly some form of affection between the two. But yet, they are tasked with an ongoing mission that's violent in nature, and it's something they're unable to stop. However, with the themes of love, pleasure, and violence being interchangeable, this can pose as both a challenge and opportunity for 2B and 9S's ongoing battle.