What good fortune!   From the very beginning of your life, you have had the privilege of sharing your entire existence with someone just like you. A playmate, a confidant, a partner in crime... Sure, one of you is older by a few minutes but that's never stopped you from forming the closest of bonds. Your twin means everything to you and you cannot... No, will not, consider even for a moment that your life could go on without them.  

Among other things, You are somebody's twin brother or sister. Whether you and her (or him) are fighting inseparably side by side, or you alone search for that lost connection, somewhere in another kingdom, mouthing your sibling's name between prayers of reunion. Work with your DM (and potentially another player, in case you choose to play as said player character's twin) on the nature of your relationship with your sibling: how do you keep in touch, if at all? What was life like growing up with or without your twin? Did something separate your character from her twin, and if so, what might have gotten between them? Are you identical twins or fraternal twin siblings? Do you have lot of things in common with your twin sibling, or disagree with him or her more often than not? How much do you tell each other, given the option? Do you two get along well? Do either of you picture the other as an "evil twin"? Alternatively, your relationship with your twin might be bitter, as you compete against your sibling and struggle for your parent's affection.

 

Skill Proficiencies:  

That's just spooky...

 Others tend to find both your relationship and similar mannerisms pretty unnerving. As a result, you have advantage intimidating or persuading if your twin is close by (within range of your walking speed). 

 Twinitiative

At the start of combat, the twins may take the lowest of both twins' initiative rolls for both twins to use for combat if they are within range of their walking speed.  The twin with the higher roll will go first in the sequence.  In the case of a tie in initiative, the twins can decide during their turn who goes first each round.  Twinitiative allows the twins to attack at the same time without sacrificing extra attacks or bonus actions in place of a reaction, with the penalty of being slower on the draw.  It also helps for tactics and concentrated attacks.

 

Languages: Twin Telepathy

 It is a very common occurrence that a pair of twins develops their own language over time, that only the two of them can understand. The connection between you and your twin is so strong that you have transcended this phenomenon and are able to communicate with one another without words - you can talk inwardly to each other with your thoughts.  In some instances, at the DM's discretion, the telepathy may not function (e.g. unconsciousness, great distances, or planar separation).

Suggested Characteristics

The implied reference point of picturing a twin brother or sister's appearance is that of the original character concept it was born with. Even among identical twins exist a few subtle differences in appearance like different hair, clothes, eyes, smile or even physique or gender that help set them apart to an observant eye. Making a fraternal twin sibling might rather have you looking to your twin for subtle resembling features, as opposed to differences.
In addition, a special quality like strangely colored irises, a scar or tattoo, jewelry or speech impediment or the like can also splash a unique color to your character.

d8	Personality Trait
1	It's uncanny how my twin and I are always/never on the same wavelength, to the irk of those around us.
2	All I ask for is not to be compared to my twin brother or sister.
3	I would pay gold to watch my sibling(s) struggle in a tough situation.
4	I have made peace with my twin's shortcomings; perhaps I'd even welcome more of them, if and when such arise.
5	I'm usually very mature and calm, except when my twin is around.
6	I was spoiled before starting my adventuring life. Can you expect any less from our parents' favorite child?
7	I never knew I had a twin sibling before, and even now, I'm not used to the idea yet.
8	If I learned anything from my upbringing, it is that I need to be devious and conniving to get what I want.

d6	Ideal
1	Responsibility. It's my job to keep those closest to me out of trouble. (Lawful)
2	Family. Blood is thicker than water, without question. (any)
3	Dominance. I am the better sibling, and there's nothing I won't do to prove it. (Evil)
4	Greater Good. the most important thing is to be someone others would strive to be. (Good)
5	Vengeance. I have been greatly wronged throughout my entire life. It falls upon me to bring justice to the family. (Evil)
6	Independence. I want others to see that I am my own personality, and not an extension of my twin. (Chaotic)

d6	Bond
1	My sibling and I had a feud when we were younger, and I just want (or don't want) to leave it behind us.
2	Somewhere, during our time apart, I have committed a crime using my twin brother's or sister's name as an alias, I hope he or she would forgive me for it.
3	My twin must not find out how I covet his or her birthright to a position of power, even if he or she did not know of the title.
4	My twin sibling and I each have fragments of a mystic artifact ingrained within our souls, and I need his or her aid to extract the pieces.
5	I have a friend or lover who I choose to keep secret from my twin, knowing well enough that he or she is everything my sibling dislikes in a person. I aim to tell my twin eventually, though.
6	I work to great lengths so my sibling does not discover the truth about me... about us.

d6	Flaw
1	My decisions cause only trouble or pain for my twin (or my family), no matter what I do.
2	I have this unshakable fear, that my brother's or sister's allies aim to turn my twin against me.
3	My ideas for how to win my twin brother or sister's affection or safety are signals of my deepening descent into madness.
4	Whether it's because of the shame, the hatred or a traumatic event, I can't even bear to stare at myself directly in the mirror; Much less so at my twin, for that matter.
5	My twin? As my equal? Don't make me laugh!
6	I am unable to cope with my twin's disapproval or absence.