Bonding Over Missing Parents

Alice has just met Bob, and although the two of them come from very different backgrounds, they have one very important character trait in common: Both of them have missing parents. It doesn't matter what the circumstances (although the circumstances might be very similar if only for plot convenience); one might have a Missing Mom and the other a Disappeared Dad, or maybe both of them have parents who were killed due to war violence, or maybe one or both of them never even knew their parents. Whatever the circumstances, this link between them is key to their relationship together.

Often, the "bonding" part of this trope is done in a scene designed to stir up some emotion, with the two characters alone together, maybe sitting on a hillside or something, and something in the conversation leads them to talk about their parents.

Variations of this can cover any missing relative, or anyone who is sufficiently close enough to a character that they consider each other family. A relatively common twist on this trope is a revelation that the two characters are missing the same exact person. (e.g.: Luke and Leia learning that they are brother and sister.)

Examples of Bonding Over Missing Parents include:

Anime and Manga

In Mahou Sensei Negima, Negi and Asuna smooth out some initial conflict by bonding over their mutual parentlessness.
In the Nanoha Striker S supplementary manga, Teana starts showing much more understanding and care (in her own way) for Subaru after learning about the death of Subaru's mother in the line of duty, since she herself lost her surrogate-parent brother in a similar fashion.
A one-sided version in One Piece. In Usopp's introductory arc, Usopp, whose father had left his family to become a pirate and whose mother died of illness sometime afterward, befriends Kaya to help her get over the recent death of both of her parents, having first-hand experience of what she was going through. However, the usage of this trope isn't brought up until the arc is over and Usopp has left the island both of them were living on, and not by Usopp himself, but by Kaya's butler, who had learned Usopp's back story from the other villagers and figured it out.

Comic Books

Batman and the first two Robins bonded over their parents being violently killed in front of them.

Film

Footloose has a minor variation; Ren has a Disappeared Dad, and Ariel has a dead brother. They bond over this while screaming at a train. Ren and Reverend Moore (Ariel's father) also later have this scene together for a moment to establish that they are Not So Different.
Jumanji had Alan Parrish talking with Judy and Peter about their dead parents after learning his own parents have died.
One of these leads to The Reveal in The Parent Trap.

Literature

In one story of TKKG, two side characters do this.
In A Little Princess, Sarah calms the tantrum-throwing Lottie by bonding with her over their dead mothers.
The lead characters of Left Behind: The Kids bond together when their parents are either raptured or killed.

Live Action TV

One of the (many, many) bonds Mulder and Scully share in The X-Files is that both lose their respective fathers early in the series.
On Bones one of the first times Brennan was shown sympathising with another human being was when she revealed to a Foster Kid (whose foster brother had been killed) that she had been in the foster system too.
Merlin and Gwaine do this over their deceased fathers.
- For that fact, same could be said about Merlin who has a Disappeared Dad and Arthur who has a Missing Mom.
On Leverage Parker and Hardison have this during "The Stork Job".
In Supernatural both Castiel and Dean have fathers who abandoned them at some point in their lives. However Castiel's Dad is God.
In the first season of Glee, Kurt and Finn bond over Kurt's dead mom and Finn's dead dad, Kurt telling Finn that he sometimes lies down on the floor next to the dresser that still smells of his mom's perfume, and Finn showing Kurt some of his dad's things. Later, Kurt's dad and Finn's mom marry each other.

Video Games

The first Mega Man Star Force game has its fair share of this. Geo and Sonia bond over his missing dad and her dead mother. Geo and Pat have an almost-romance over Geo's missing dad and Pat being an orphan.
A Dummied Out conversation in Mother 3 has Kumatora and Duster muse about how none of the three human protagonists have mothers. Kumatora is genuinely an orphan, and Duster's mother seemingly died before the White Ship departed. Likewise Hinawa isn't even confirmed to be Lucas's genetic mother.

Web Comics

El Goonish Shive has Tedd and Susan finding a common ground.

Western Animation

Avatar: The Last Airbender loves this trope, usually with regard to Katara and Sokka's mother (who was killed in a Fire Nation raid of their colony):
- Katara and Haru, as Haru's father was taken away from his family by the Fire Nation due to laws against earthbending in his town. This leads Katara to help him and his dad, who is on the same prison rig as Haru is taken to, when he gets arrested for the same reason.
- Jet attempts to use this to manipulate Katara and Sokka into destroying a town occupied by the Fire Nation.
- Katara and Zuko, during their Chained Heat scene in the season 2 finale; when Katara mentions that the Fire Nation took her mother away, Zuko says, "That's something we have in common," and tensions cool for a while, until Zuko's "Friend or Idol?" Decision between siding with Azula or making a Heel Face Turn leads him to completely ignore all that bonding and side with Azula.
In Young Justice, Zatanna's mother isn't in the picture, and she suddenly loses her father when he sacrifices himself to become the vessel of Doctor Fate, and prevent him from keeping hold of Zatanna. While Robin has been forbidden by Batman to reveal his secret identity (and, with it, the fact that his parents were murdered when he was nine), he's usually the one shown grieving with or comforting her. He had a crush on her already, but the show seems to imply that his empathy was what pushed them towards their kiss in the finale.