The characters' abilities can be combined in various ways. For example, a Naruto character could use their chakra to enhance the physical strength of a Resident Evil character. So, if Leon Kennedy was given some chakra by a Naruto ninja, he could become even more powerful in combat. Also, a Resident Evil character's knowledge of weapons and survival skills could be used to help Naruto characters in the Resident Evil world. For instance, Ada Wong could teach Naruto characters how to use guns more effectively. In terms of the harem, a female character's charm or intelligence from either world could be used to influence the group dynamics. A Resident Evil girl could use her scientific knowledge to help Naruto characters understand the viruses better, and in return, they could teach her about chakra.
The abilities can be combined quite interestingly. Take the case of Naruto's shadow clone jutsu. A Resident Evil character could use this jutsu to create clones of themselves to distract enemies while they set up traps or find a cure. In reverse, a Resident Evil virus could be used in a way to enhance a Naruto character's transformation abilities. For example, if a character like Kiba were exposed to a carefully controlled amount of virus, his transformation with Akamaru could become even more powerful. In the context of the harem, the different characters' abilities in terms of intelligence and combat could be pooled together. A smart Resident Evil character like Claire Redfield could strategize with a Naruto character about how to deal with a new threat. And the combat skills of Naruto characters could be enhanced by Resident Evil weapons. For example, a kunai could be combined with a Resident Evil firearm to create a new and deadly weapon.
Well, in a Naruto Resident Evil harem fanfic, one way to combine the abilities is through a new form of jutsu. A Naruto character might develop a jutsu that can mimic the effects of a Resident Evil virus. For example, Sasuke could create a jutsu that gives the appearance of a T - Virus infection but is actually a form of genjutsu. This could be used to deceive enemies. On the other hand, a Resident Evil character could use their virus - related knowledge to create a new type of weapon that is powered by chakra. Regarding the harem, the female characters could combine their medical skills. A Naruto kunoichi and a Resident Evil medic could work together to develop a new healing method that uses both chakra and Resident Evil - style medicine. Also, a character's speed from Naruto, like Rock Lee's, could be combined with a Resident Evil character's stealth to create a new combat style for the group.