Multi-Skill

Multi-Skill (多才能力 (マルチスキル) ) is an ability initially derived from Kiyama Harumi's connection to the Level Upper Network, where she is able to use multiple abilities of those who used Level Upper.

Overview
Users of the Level Upper are connected to the Level Upper through the use of their AIM dispersions and forcibly alter their brainwave patterns to match that of Harumi. Because of the network created by the similar brainwave patterns of the espers, they are able to borrow vast amounts of processing power, considerably heightening the esper's powers and giving abilities to those who have yet to manifest any noticeable abilities.

Harumi – who controls the network, can use the powers of those connected to the Level Upper Network. It is similar to Dual Skill – the ability for an esper to have more than one ability, though according to Harumi, a person who has Dual Skill is an impossibility – as it has been concluded that an esper can only have one ability – and seem to indicate that Multi-Skill is not like Dual Skill.

The main difference between Dual Skill and Harumi's Multi-Skill is that Dual Skill solely relies on the esper's Personal Reality, while Multi-Skill relies on several esper minds that are connected to the network, which makes Harumi simply borrowing the abilities of the espers in the network.

Effects
As its name states, Multi-Skill allow Harumi to use the powers of the espers' connected to the network. It is shown that Harumi can use more than one power at the same time. One noticeable effect in using Multi-Skill is having the eyes sclera's turn red along with the iris turn yellow, though there is no explanation as to why this occurs.

Level Upper Arc
The only instance of Multi-Skill is during the Level Upper Arc, where Harumi uses it to decimate the Anti-Skill that is after her. Using Multi-Skill, Harumi is able to keep Misaka Mikoto – the 3rd Academy City Level 5 – in a difficult situation, as her attacks are overwhelmed by Harumi's attacks as well as the unpredictability of her having many different abilities. Harumi is later defeated by a surprise attack by Mikoto and later allows the manifestation of the AIM Burst due to the subsequent overrunning of the network after their battle.

Daihasei Festival Arc (Railgun)
Kihara Gensei obtains this by using Level Upper on a group of specifically chosen "volunteers" to give himself their powers to aid in his experiment and his siege of Exterior.

Trivia

 * The anime adaptation of the manga only colored one of Harumi's eyes red. Coincidentally, the AIM Burst's eyes have a slightly similar color scheme to Harumi's eyes when she uses Multi-Skill.