Sheldon Oswald Lee is Jenny's geeky friend and admirer.
Sheldon made his debut appearance in "Attack of the 5½ Ft. Geek".
Sheldon is a 5-foot 6-inch tall (as noted in "Attack of the 5½ Ft. Geek" ) teenage boy. He is 16 years of age through seasons one and two, but is technically 105 after season 3, episode 2, Good Old Sheldon. Sheldon is depicted with greasy black hair, and two specks on his cheek that could either be freckles or acne. His hair has two cowlicks at the crown of his head, and he is shown to have crooked teeth and a slight overbite. He is usually dressed in blue-gray pants, a white T-shirt, and an auburn hoodie.
Sheldon is a "geek" stereotype, heavily invested in both science and comic book culture. His major character trait is his obsessive crush on Jenny. It is shown in the episode Designing Women that he has a shrine for her, and he admitted in Agent 00' Sheldon that he stalks her. Despite the extent of his affection, he never is able to get Jenny to go out with him, and the two are simply close friends. Whilst normally sweet-hearted and intelligent, Sheldon easily becomes jealous of any boy who shows affection for Jenny, and is somewhat possessive of her.
Sheldon is also a budding engineer, inventor, and scientist, and often invents useful gadgets for Jenny in the hope of impressing her. He also created a robot superhero, The Silver Shell, in an attempt to show Jenny that robot boys would not be a good match for her — but this backfired when she actually fell in love with The Silver Shell. Jenny did become frustrated with Silver Shell when he inadvertently ruined her first dance in Dancing With My Shell, but later, in A Pain In My Sidekick, she seemed to have forgiven him for this mishap, as she joined forces with him to protect Tremorton by fighting off a Moleverine.
Sheldon seems to like reptiles, as seen in Shell Game when he tries to invite Jenny to a reptile convention on a date. He is a cat lover as well, as shown in Love 'Em or Leash 'Em were he reveals that he dresses up in a cat costume when he attends the "Feline Fan Convention". He is also a very skilled swimmer, according to Brad.
Jenny sees Sheldon as a very smart and sweet boy and a faithful friend who is always eager to lend a her a helping hand, although he can sometimes get himself into trouble. Jenny also relies on Sheldon to fix malfunctions that her Mom is either too busy to or unable to fix.
Despite seemingly not returning Sheldon's deeply romantic feelings for her, Sheldon remains one of Jenny's closest friends whom she often hangs out with. Jenny has even shown genuine affection for Sheldon (even if it is platonic rather than romantic).
In A Robot For All Seasons Jenny kissed Sheldon on the cheek while holding up mistletoe as a heartfelt thank you for Sheldon being the only one in Tremorton to never lose faith in her, thus saving her reputation as a superhero, and freeing her from Todd Sweeney’s evil mind-control device. Jenny kissed Sheldon a second time in Agent 00' Sheldon after he saved her and all of the robots in Tremorton from a top-secret organization bent on eradicating them.
Throughout the series Sheldon has saved Jenny quite a few times and helped her fight off villains, thus she became deeply grateful for Sheldon's loving friendship and utmost loyalty to her.
In an interview with series creator Rob Renzetti, he made a comment about bringing Jenny and Sheldon together as a couple, and admitted he was rooting for him.  In a later Q&A, Renzetti clarified that he had no long-term plans for Jenny's love life, which he considered a minor part of the show anyway, and being a nerdy person himself, he sympathized with Sheldon. 
Brad is one of Sheldon's good friends. They enjoy hanging out both inside and outside of school, and going on adventures together, usually with Tuck and Jenny.
Tuck and Sheldon are rarely seen talking to each other on a regular basis. This may be due to Tuck being much younger, and the two being in different levels of school.
However, in A Pain In My Sidekick, Tuck convinced Sheldon to make him a robot suit to become "The Tin Can" because he desperately wanted to be The Silver Shell's sidekick. Tuck orders Sheldon around in Tuckered Out, riding him like a horse and complaining to him when things go wrong. Sheldon ultimately gets revenge by helping Brad and Jenny play embarrassing home movies of Tuck to the entire town. Sheldon sells Tuck his time machine in The Price of Love (in order to make enough money to go on a pretend date with Pteresa to make Jenny jealous).
Both Sheldon and Dr. Wakeman care deeply for Jenny, and show concern for her well-being when she is in trouble. In Escape from Cluster Prime, Sheldon and Dr. Wakeman join forces and work together to stop Vexus and save Jenny by creating a portal device to transport them into Vexus' starship and battle her. When Sheldon was 75 years old in Good Old Sheldon, he flirts with Dr. Wakeman, much to Jenny's disgust. It can be inferred that Sheldon's crush on Jenny may stem from an older crush on/interest in her mother, or vise-versa.
Sheldon dislikes Queen Vexus as Jenny does. However, in Designing Women, Vexus tricks Sheldon, disguising herself as a cute and flirtatious robot girl named QT-2 in order to take XJ-9's Master Plans from him, as he had stolen them from Dr. Wakeman’s lab to find a way into Jenny’s heart. Sheldon developed a brief crush on Vexus in this cleverly disguised form. When Sheldon gets "QT-2" a drink, Vexus removes her disguise to sneak out and take the master plan with her. Sheldon reveals to Jenny what he did, and helps Jenny by giving her one of his inventions to defeat Vexus, retrieving and returning the master plan to her mother.
Sheldon is especially devoted to keeping Jenny safe from and helping her fight against The Cluster's relentless attacks. He dislikes them as Jenny does.
Due to being polar opposites, Sheldon and Don Prima are sworn enemies. Don Prima is a popular, handsome social butterfly, as opposed to Sheldon, who is a geeky social outcast.
In The Price of Love, Sheldon paid Pteresa to pretend to be his girlfriend for a brief time (in order to make Jenny jealous), but he truly could not stand her cruel, bullying, greedy and manipulative ways.
- Jenny has affectionately kissed Sheldon on the cheek at least two times, in A Robot For All Seasons and in Agent 00' Sheldon.
- In The Price of Love, it is revealed that Sheldon created a functional time travel machine, but sold it to Tuck for two cents.
- In the same episode it is shown that sometime within twenty years, Sheldon becomes a billionaire.
- Sheldon bears a resemblance to the Japanese family in Speak No Evil.
- In the Russian dub of Season 3, Sheldon is voiced by a woman.