Alex Jensen is a Caltech Physics doctoral student and Sheldon Cooper's assistant. Alex is often required by Sheldon to perform menial tasks, as opposed to assisting with his research. She often uses a very wide-eyed expression when having to deal with the gang's unconventional behavior. She displays a strong attraction for the initially oblivious Leonard, much to the chagrin of Penny and with interference from Sheldon.
A native of Des Moines, Iowa, she graduated summa cum laude with a concentration in theoretical physics from Stanford University. Inspired by her father, a SETI astronomer, her dissertation concerns looking for Trojan asteroids at Earth's L5 Lagrange point. She gave up an opportunity at Fermilab to come work for Sheldon.
She is introduced in "The Higgs Boson Observation" (S06E03) when Sheldon hires her. Alex happens to be quite skilled with flattery, shown when she uses it to persuade Sheldon to hire her, even though immediately beforehand he insists that flattery does not work on him. When Leonard and Raj stop in to visit Sheldon, they meet Alex, whom Raj immediately falls for. Later, Alex meets Penny, whom she mistakes for Leonard's assistant. Penny becomes jealous immediately after Alex says how talented he is and Penny takes Leonard off to the bedroom.
Alex is next seen in "The 43 Peculiarity" (S06E08), where she shows an obvious interest in Leonard, though Leonard remains oblivious to her flirting and signals of attraction. She compliments him and tells him that girls must hit on him all the time since he is cute and funny. This attention is further explored in "The Egg Salad Equivalency" (S06E12) when Alex suggests that they could discuss the physics lecture she was attending over dinner. Her suggestion really startles Leonard. Although Leonard remains faithful to Penny, her offer does not stop his ego from inflating for the rest of the episode.
Sheldon, however, is jealous that she is showing any form of attention to any physicist besides himself, and attempts to discourage her advances and keep her focused on his needs. His efforts backfire spectacularly when she takes offense to his numerous condescending and demeaning remarks about women and their biological urges, causing her to file a sexual harassment complaint with a Human Resources administrator. The HRA's attempts to show Sheldon that his actions were offensive seem initially successful, with Sheldon later revealing to Alex that he is required to complete an online sexual harassment seminar; however, as he unaffectedly deems his time far too valuable, he asks Alex to take it in his place.
Alex Jensen next appears in "The Tangible Affection Proof" (S06E16). With Valentine's Day approaching, Sheldon has Alex buy his gift for Amy. Sheldon gives Alex two thousand dollars in various crumpled bills that he finds in his pockets and his desk, and instructs Alex on Amy's interests (monkeys and the color grey) so that Alex can find an appropriate gift for Amy. Alex buys a miniature harp music box that plays Amy's favorite song, a Canterbury Tales' map and a neuron cell drawing drawn and signed by famous neuroscientist Santiago Ramón y Cajal; all items purchased based on Amy's interests. Sheldon likes the drawing so much that he keeps it for himself.
- Alex has made no appearances after Season 6 and no mention of her fate, thus it's unclear what became of Alex after her last appearance, and it's also unknown if she resigned or was fired from being Sheldon's assistant.
- Alex's presence was to force Penny to realize that other smart and pretty women are also interested in her boyfriend Leonard.
- Alex's last known appearance in "The Tangible Affection Proof" is the only episode where she doesn't display her crush on Leonard.
- The last few gang members that Alex still hasn't met are Bernadette, Stuart and Emily since her only appearances in those four Season 6 episodes (1: S6E3, 2: S6E8, 3: S6E12 and 4: S6E16).