First he is seen outside in the rain when Sen is dumping water outside during work, so she leaves the door open for him to enter. Because of this No-Face takes a liking to Chihiro and offers her many gifts including bath tokens which came in handy.
First, while in the bath house, in one of the tubs, he swallows a green frog for trying to take his gold. After that, he is eventually overcome by the greed of the bathhouse employees and then has a frog man wake everybody up and eat all the food while taking a bath. Even later, he then swallows that man for "smiling" and a slug woman. This is why Yubaba is furious about his presence and doesn't want him in the bath house anymore. Much later, he demands the presence of Sen, whom Yubaba pushes into that room.
Back there, No-Face tries offering Sen a delicious dish and some gold, the latter of which he keeps persisting in offering it, becau No-Face is a lonely spirit who follows Chihiro after having developed an interest in her sincerity. His vocabulary consisted of grunts and moans as opposed to coherent words. Not knowing much about The Bathhouse or about other spirits in general, No-Face learned by example and adapted to his surroundings.
Exposed to the corruptive thoughts and greed of the workers, he quickly grew to encompass their personalities, hoping that his endeavor to be like those around Chihiro would eventually garner her affection for him. While this ultimately brought the opposite effect, Chihiro was somewhat true to No-Face after she invited him and accepted help from him.
No-Face becomes obsessed with Chihiro, and wants her to see her and her only. He becomes extremely volatile after being fed the Unnamed River Spirit's emetic dumpling by Chihiro, and, while fleeing from the obviously now-hostile monster, she calls out to him twice to follow him. She accepts him as a co-journer into the train on her way to Zeniba's cottage after he calms down. At this stage No-Face is no longer aggressive and is quite docile, obeying Chihiro's orders like "sit and behave yourself".
As a monstrous out-of-control being, No-Face exhibited all the negative traits of those it had swallowed (most significantly, Aogaeru), becoming a brash, arrogant, loud and selfish creature whose obsession with Chihiro proved troubling for everyone.
Towards the end of the film No-Face meets with Zeniba, another witch who works alone, in contrast to Yubaba who runs her bathhouse with a complex hierarchy. Perhaps attempting to match the rules of the new tiny society, No-Face starts spinning and knitting and appears proficient in that. Happy with his new environment, he accepts Zeniba's proposal to stay with her as an apprentice.se he's "not giving it to anybody else". Sen, despite intending to save the last of her herbal ball for her parents (whom Yubaba had turned into pigs), feeds it to No-Face, causing him to vomit. When Yubaba demands that the nonsense stops and zaps a ray at him, he only vomits a bunch and it lands on her as he chases after Sen. At the last floor, he vomits the frog man and slug woman, vowing to get Sen for it. Outside, Sen has been calling for him to follow them outside the bath house, and in the flood from all that rain, he vomits the little green frog up, and the latter swims away. Now out of the bathhouse, No-Face returns to his calm normal self but was still fond of Sen. No-Face follows after Sen slowly, and Rin threatens him with him paying if he even touches Sen. He does, however, behave himself on the one-way train and goes with her and her mouse and mini blackbird friends (actually Boh, Yubaba's baby, and the Yu-Bird, respectively, transformed by Yubaba's sister Zeniba).
Now at Swamp Bottom, where Zeniba's house is located, No-Face follows Sen and her little friends to the house, with a one-footed hopping lantern as their guide. Eventually, No-Face shows Zeniba his spinning thread, and when Sen, the mouse and blackbird leave with Haku (as a dragon), No-Face stays behind to help Zeniba.