A person you date to make yourself seem more desirable
I found out Luke was just using me as a date ladder to get to Claire.
|Outkick your coverage||To have a romantic partner who is much more attractive than you|
|Trophy wife||An attractive wife of an older man|
|Trophy husband||An attractive husband|
|Caspering||Ending a relationship in a friendly, but confusing way|
|Cuffing||Linking up in a relationship with a person|
|Layby||Flirting with a person when in a relationship|
|Stashing||Dating a person but unwilling to publicly acknowledge it|
A date ladder is someone you date primarily in hopes of attracting someone more desirable. Using an SO in this way can also be referred to as "climbing the dating ladder" - because you are attempting to reach the more attractive mates at the top of the ladder.
For example, if you're really into a girl named Matilda, but you think she's out of your league, you may attempt to use Matlida's friend Jean as a date ladder. In theory, this may lead Matlida to see what she's missing by not dating you. In reality, however, this will likely just make your life more complicated ...