It's unclear whether or not he is human, as he is described as having a tongue that lolls out of thick purple lips and hissing. Despite his unnerving description, Cecil admires him greatly and aspired to take his place as host on the show one day. Cecil describes Leonard as being "super nice" and very encouraging to him and his goals, including Cecil in the radio show experience during his internship at the station.
His voice is described as being "high-pitched and grating like sandpaper, just the way radio voices should be".
Leonard's tenure as host of Night Vale Community Radio has never been specifically identified, but was lengthy enough to make Cecil impatient for his retirement. Leonard did eventually retire some time during World War II, after which Cecil immediately took over Leonard's job as full time host of Night Vale Community Radio.
In Best Of?, Leonard (at least according to one of the tapes) was revealed to have met a grisly demise sometime during the Clinton Era, with Cecil announcing it distraught - what this means is uncertain, though Leonard merely brushes the tape off, saying he "doesn't like it" and that his "day was better before hearing these".
In The Broadcaster, it is revealed that Leonard died after being hit by a truck while attempting to cross the street.
Leonard's style on the show was slightly different from Cecil's. He started with a short intro (presumably changing from show to show, though evidence so far shows him using the same intro repeatedly) as Cecil does, however his catchphrase afterwards was "Greetings from Night Vale", rather than the familiar "Welcome to Night Vale". The intro Cecil uses in Cassette, which is at least modelled after one of Leonard's, was "The sun is actually cold. It’s cold and empty and all is lost. Greetings from Night Vale". His endings also varied from the current show - whereas Cecil ends his broadcasts with "Goodnight, Night Vale. Good Night." , Leonard signs off with "See ya, Night Vale. See ya."
Leonard came out of retirement to cover hosting duties at Night Vale Community Radio while Cecil was away on vacation during Best Of?.