Klaus (Robert Sheehan) was the first member of the Hargreeves siblings with the power to see and communicate with Ben (Justin H. Min) in The Umbrella Academy, but he doesn’t enjoy sharing that fact with his family. Also referred to as “Number Four,” Klaus serves as one of Reginald Hargreeves‘ (Colm Feore) adopted children who he raised to form a group of superpowered crime-stoppers. He and Ben were born under unique circumstances on October 1, 1989, along with his fellow adopted siblings Luther (Tom Hopper), Diego (David Castañeda), Allison (Emmy Raver-Lampman), Five (Aidan Gallagher), and Vanya (Elliot Page). Ben died before the siblings reached adulthood, but the character is still present in the series thanks to Klaus’ powers.
Each member of the Hargreeves siblings harbors drastically different powers. Klaus, in particular, is capable of speak to the dead, but his gift goes much deeper than that. As a trained medium, Klaus communicates with spirits who have unfinished business in the afterlife. Granted, this power only works when Klaus is sober. Considering he’s afraid of ghosts and interacting with spirits, Klaus suppresses his mediumship with drugs and alcohol. Besides being a medium, Klaus also has powers of conjuring spirits to manifest into the real world, also known as evocation.
Klaus’ use of evocation is how he brought back a version of Ben following Reginald’s funeral in The Umbrella Academy season 1. Ben died years before the events of the debut season, but Klaus manages to reunite with his brother, allowing the character to participate in the various apocalyptic threats. Though Ben often followed his brother around to steer him in the right direction, Klaus didn’t reveal Ben’s presence to the others. He either hid the truth from his siblings or outright lied, like Klaus’ claim that Ben didn’t time travel to the ’60s. Diego and Vanya eventually got special reunions with Ben in season 2, but series creator Steve Blackman previously explained (via Reddit) why Klaus hides the fact he can see Ben. Blackman stated Klaus “is selfish and knows he is the conduit to Ben’s connection to the world” before adding Sheehan’s character never wanted to share his brother with the others. There may also be another side of Klaus’ secret that connects to Reginald.
Klaus never intentionally meant to bring back Ben. Instead, he attempted to summon Reginald at the request of his siblings. Upon failing to do so, the series revealed Ben’s presence, but Klaus elected to keep that to himself. He claimed that he was too high to successfully use his conjuring powers, in addition to Reginald being too stubborn to return. Some of that could have been true, but it was likely Klaus didn’t want to face his father. Each member of the Umbrella Academy had issues with Reginald based on how he treated them their entire lives. As the family rebel, Klaus was never keen on reuniting with the figure he thought ruined his life.
After a near-death experience, Klaus had the chance to interact with Reginald in the afterlife. During the encounter, Klaus was told he only scratched the surface with his powers. Some of Klaus’ expanded potential was explored in The Umbrella Academy season 2 with his ability to host spirits for longer durations. Still, Klaus decided not to share the true potential of his powers. He may have been afraid his siblings would force him to use his abilities more often. By not fully overcoming his fear of interacting with ghosts, it made sense why Klaus still struggles to embrace his gifts. That could change now that the original Ben is gone and more of the truth is out.