In 2151, Leonard was part of a group of Starfleet officers who conferred with VulcanambassadorSoval at Starfleet Medical, regarding the disposition of Klaang, a Klingoncourier who had recently crash landed on Earth. Leonard expressed annoyance with Soval, who recommended allowing the Vulcans to handle the situation, responding that the incident had occurred on Earth soil, and therefore should be Starfleet's responsibility. Later in the discussion, Leonard mistakenly identified Klaang as a "Klingot" to CaptainArcher. (ENT: "Broken Bow")
Leonard was named after Leonard Nimoy, the Star Trek: The Original Series actor who played Spock. As evidenced by the second draft script of "Broken Bow", Admiral Leonard's first name was originally "Henry". Although the character is referred to on-screen as "Dan", it is unclear – from evidence established in canon – whether this is meant as a shortened form of "Daniel". This likelihood was confirmed in the revised final draft of the script, however. 
In a character breakdown sheet that Paramount sent to talent agents upon seeking an actor for the part, Admiral Leonard was described as follows: "Human male. 40s – 50s. A high-ranking officer in Starfleet who serves directly under Admiral Forrest."