Reva Rain deeply affected Eren's personality as a ranker.
She was his mentor in the art of inflicting pain and misery on others. She also showed him with her actions that staying low was paramount when it came to conducting operations like these.
Reva taught him many things and made him realize even more. The lessons she imparted to him were bone-deep– something he wouldn't be able to forget even in the current timeline.
Experiences can be classified as good or bad. However, the knowledge gained from those experiences will always hold some value. It depends upon the individual how they apply the knowledge gained.
Eren couldn't use what Reva Rain taught him to its fullest potential in the last timeline. However, the skills he had developed back then had become his assets in the current timeline.
