Leonard Tobias Tennyson, more commonly known as Len, is Ben’s second cousin, and a main character of the fanfiction series Ben 10: Hero Force.

He is first introduced in the episode Genealogy, where he reunites with the Tennyson family and then, throughout the series, learns to use his energy conversion powers, to become a true hero.

It should be noted that Len is half human/half Malverosapien.


Len has dirty-blonde hair and eyes that change color, but are usually green (in Hero Force his eyes appear to be of a darker green). In Hero Force he wears an orange t-shirt with a gray jacket over it and a pair of grayish-black jeans.


For most of the series, Len is portrayed as a mature, modest boy who would occasionally give credit to others he does necessarily deserve. He also had a complex love-hate relationship with his second cousins Ben and Gwen.

Len is good-hearted, and his actions are motivated above all by a strong and sincere will to help other people. He will stop at nothing to protect his family or anyone in danger. His realistic nature tends to make him act discerning.

He is often said by many other characters to be extremely intelligent.

Powers and AbilitiesEdit

Energy ConversionEdit

Len is descended from a strategic alien race known as Malverosapiens from the distant planet Malveros. These aliens have the unbelievable power and ability to manipulate and convert any type of energy.

In the episode Lend a Hand, Len's powers are confirmed as being alien in origin; inherited from his father Lione, a member of a race of beings known as Malverosapiens, from the distant planet called Malveros. Lione tells Len that, like himself, Len is a Malverosapien and thus is able to convert any energy. Lione offered to take Len back with him to his home planet of Malveros and train him in the full scope of the powers of a Malverosapien, but Len prefers to remain on Earth with Ben and Gwen.

Limited TelepathyEdit

Len is also able to mentally communicate with people within a 200-mile radius, allowing him limited clairvoyance, but only when in his Malverosapien form. In his human form, he can only use energy conversion.


Len is shown to be quite intellectual and knowledgeable (if more so than Gwen). Before discovering his powers he would often use his "brains" to help Ben’s Team.


  • Len's middle name is Tobias. His cousin Gwen in the Hero Force episode "Nine Lives" proves this.

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