Someone whom you employ to break wind in your stead (da word can also refer just to said "proxy" whizzpopper itself) on occasions where you'd thought dat you'd "really had a good one all ready", but then it turned out to merely be a "false alarm". Be sure to frequently feed said butt-splutter supplier a sizeable portion of baked beans, of course, so dat he can always be reliably "primed 'n' prepared" whenever a supplementary fart is required.
Da BFG never would need a substitoot --- actually, doubly so in his case --- because not only did he never have any trouble with copiously "blowing his trumpet" after drinking frobscottle, but as he told da Queen of England (and then noisily demonstrated said fact), he could still rip one "all on his own" --- i.e., regardless of whether or not he'd consumed any gas-producing foods in da recent past.
The significant other of the substitute person may not "have enough time" or "be too busy" for a relationship, or they may only be interested for the purpose of sex, money, status, or general need of someone to talk to during times of stress. The substitute person generally won't get the emotional fulfillment from the experience that he/she may crave, and will feel unimportant yet somehow will continue on out of hope.
This can only end in tears for the substitute person.
A quote from the low-budget sci-fi/fantasy film The Dungeonmaster, popularized by Adam Savage of Mythbusters, this quote basically means "you may be technically right, but you're not changing my mind." This can come off as stubborn, but it really doesn't matter, since you don't care about the technical "facts".
Person 1: Ugh this character's design is so terrible and pointless.
Person 2: Well actually, did you know that it's design comes from goes on to prove person 1 wrong
Person 1: I reject your reality and substitute my own.
Person 2: -sigh- Okay then, moving on...
Kirito: I reject your reality and substitute my own.
Kayaba: "NICE!! Dungeonmaster!"
Kirito: "What? No! Mythbusters... What the hell is Dungeonmaster?"
Kayaba: "Oh. I was so happy there for a second."
Any replacement for coffee. Energy drinks are commonly used as a coffee substitute. Tea can be a coffee substitute for some people, as well as caffeine tablets.