Zack Johnson

Extremely naive and simple-minded – Zack's low intelligence is a major source of comedy. His questions are often absurdly funny, like worrying that a laser experiment might "blow up the moon." He has even less understanding of sarcasm than Sheldon.

Kind and gentle, completely harmless – Unlike Penny's other ex-boyfriends, Zack never shows aggression or malice. He's friendly, always smiling, and mostly doesn't even notice when Sheldon and Leonard mock him.

Enthusiastic and agreeable – Zack almost never complains or refuses. Ask him to dress as Superman for a comic book store party — he's in. Ask him to join Sheldon's temporary social circle and sing karaoke — he loves it.

Genuinely curious about science – Despite his limited comprehension, Zack has a sincere interest in science. He listens intently as Leonard and Sheldon explain their laser experiment, even if his questions are unintentionally hilarious.

Wealthy but unassuming – As heir to a family business, Zack is actually quite rich, but he never flaunts it. His wealth only becomes truly apparent after he sells the company and retires.

Surprisingly good husband – After marrying Marissa, Zack shows a considerate side. He offered Penny a job but immediately and apologetically rescinded the offer upon realizing it would upset his wife.

Genuinely friendly – Even when teased, Zack treats Leonard and the others as friends. He joins their activities, shares fun facts he learned from the Discovery Channel, and never holds grudges.

Episodes featuring Zack typically revolve around:

Dating Penny – Zack first appears as Penny's date, and his low intelligence proves too much for her after dating Leonard, triggering a chain of events.

The accidental marriage discovery – During Thanksgiving, Penny discovers she's still legally married to Zack. She frantically arranges an annulment while Zack remains obliviously cheerful — even asking if she's sure they don't have kids.

Justice League cosplay – Zack joins the group's Justice League costume party as Superman, helping them win the best group costume prize.

Sheldon's temporary social circle – Sheldon invites Zack, Kripke, and Stuart to form a new "social group." They completely derail his plans by telling raunchy stories and singing karaoke.

The sperm donor request – In Season 12, Zack and Marissa ask Leonard to be their sperm donor. This request tests Leonard and Penny's relationship and forces Leonard to grapple with jealousy and kindness.

Amy's brief "chemistry" with Zack – In Season 4, Amy experiences sexual arousal for the first time upon meeting Zack, even seeking him out at a bar — but his stupidity quickly kills the mood.

1. With Sheldon

Sheldon: Zack, Jacuzzi is a commercial brand, hot tub is the generic term. I.e., all Jacuzzis are hot tubs, but not all hot tubs are Jacuzzis. Zack: Is that like all thumbs are fingers but not all fingers are thumbs? Sheldon: ...Surprisingly, yes. Zack: Nice! Now what exactly are toes?

2. With Leonard

Zack: Whoa, you dated Penny? Leonard: She didn't tell you? Zack: She told me she dated a guy named Leonard. Who would have thought it was you? Leonard: Who else would it be? Zack: I don't know. Somebody bigger and— Yeah, sure. Why not you?

3. With Penny (about the laser)

Zack: One question. How can you be sure it won't blow up? Leonard: The laser? Zack: The moon. Leonard: Don't worry about the moon. We set our laser to stun. Zack: Smart.

4. With Amy

Amy: I stand here before you, 130 pounds of raging estrogen, longing to grab hold of your gluteus maximus and make Shakespeare's metaphorical Beast with Two Backs. Zack: My gluteus what?

5. With Penny (about marriage)

Zack: I just think splitting up can be rough on kids. Penny: We don't have any kids. Zack: Are you sure? 'Cause you didn't know we were married until this morning.