Return to Answer

By my reckoning (some are questionable), there have been 20.5 voluntary, 11.5 forced, 8.5 deaths and 1 unknown (Ace). If you consider death a forced exit, then it's around a half forced and a half voluntary (most often for love).

Graham and Ryan leave the Doctor by staying back on earth after stopping a mutant version of the daleks in resolution of the daleks

A character Named only as Dan meets the Doctor is Season 13

By my reckoning (some are questionable), there have been 22.5 voluntary, 11.5 forced, 8.5 deaths and 1 unknown (Ace). If you consider death a forced exit, then it's around a half forced and a half voluntary (most often for love).

• Graham and Ryan voluntarily leave the Doctor by staying back on Earth after stopping a mutant version of the Daleks because they think they've traveled enough.

2 voluntary exits.

We do not yet know how Graham, Yasmin, and Ryan will leave the Doctor.

Graham and Ryan leave the Doctor by staying back on earth after stopping a mutant version of the daleks in resolution of the daleks

A character Named only as Dan meets the Doctor is Season 13

By my reckoning (some are questionable), there have been 22 voluntary, 10 forced, 9 deaths and 1 unknown (Ace). If you consider death a forced exit, then it's around a half forced and a half voluntary (most often for love).

• Susan wants to leave (she falls in love), but decides that she can't (because she has to look after her grandfather). The Doctor forces her out because he knows that it's best.
• Barbara and Ian were kidnapped by the Doctor. They leave voluntarily (the Doctor opposes it) when they find a way to get home.
• Vicki falls in love and leaves voluntarily.
• Steven leaves voluntarily in order to lead a peace effort.
• Katarina dies (sacrifices herself to save the Doctor).
• Sara Kingdom (dubiously a companion) dies by accident.
• Dodo leaves voluntarily (with no real reason given).

5.5 voluntary exits, and 2 deaths.

• Polly and Ben join the 1st Doctor by accident. They voluntarily leave when they have a chance to resume their normal lives.
• Jamie and Zoe are forced to leave by the Time Lords.  (Jamie potentially came back in "Season 6b", and his second departure is unknown; given that the Doctor still ends up exiled to Earth, Jamie likely was forced to leave again).
• Victoria leaves voluntarily to live a normal life (much the same as Martha)

• Brigadier Lethbridge-Stewart played a similar role to a companion, but the Doctor left him, so doesn't really count for this question (and he died well after this). The same is true for Sergeant Benton and Mike Yates.
• Liz Shaw never travelled in the TARDIS, but was essentially a companion. She voluntarily left UNIT (with no real reason given).
• Jo leaves voluntarily (she falls in love).

• Sarah Jane Smith is forced to leave when the Doctor is summoned to Gallifrey. She is offered the chance to re-join the Doctor later, and declines.
• Harry leaves voluntarily to resume his normal life.
• Leela leaves voluntarily (she falls in love).
• K-9 (Mk I) leaves voluntarily (to stay with Leela) and K-9 (Mk II) is given to Romana (he can only survive in E-Space). K-9 (Mk III) and K-9 (Mk IV) didn't travel with the Doctor.
• Romana leaves somewhat voluntarily (she chooses between staying in E-Space and returning to Gallifrey, rather than either and staying with the Doctor).

3.5 voluntary exits, and 2.5 forced.

1 voluntary exit.

2 voluntary exits, 1.5 forced, and 0.5 deaths.

• Amy and Rory leave unwillingly after a Weeping Angel sends Rory to 1930s New York, and Amy willingly chooses to be sent back along with him. The Doctor reading their tombstone creates a fixed point in time, so the Doctor is unable to rescue them.

• River is similar to a companion, although we don't really see much of her travelling with the Doctor. Her final appearance in The Husbands of River Song is after her official "death" in "Forest of the Dead". She is difficult to categorise due to her timeline not matching the Doctor's own.

• Like the Brigadier, Danny Pink never travels with the Doctor, but does play a similar role as a companion. He is killed when he is hit by an automobile, turned into a Cyberman, and eventually sacrifices himself to destroy the rest of the Cybermen, and sacrifices his return journey from the Nethersphere to send back a boy he killed during his time as a soldier.

• Clara Oswald is killed, then brought back between 2 heartbeats using Time Lord technology. She tricks the Doctor in forgetting about her, then leaves voluntarily for an adventure together with Ashildr in a stolen TARDIS. She will eventually have to return to her final moment so she can die.

• Bill leaves to explore the universe with Heather, voluntarily (her prior cyber-conversion doesn't seem to count as a death given the circumstances), although her only other choice was returning to Earth, not continuing to travel with the Doctor.

• Nardole is instructed by the Doctor to lead a group of people to safety, which he does. I consider this a forced exit.

1.5 deaths, 1.5 voluntary exits, 1 forced exit.

By my reckoning (some are questionable), there have been 20.5 voluntary, 11.5 forced, 8.5 deaths and 1 unknown (Ace). If you consider death a forced exit, then it's around a half forced and a half voluntary (most often for love).

• Susan wants to leave (she falls in love), but decides that she can't (because she has to look after her grandfather). The Doctor forces her out because he knows that it's best.
• Barbara and Ian were kidnapped by the Doctor. They leave voluntarily (the Doctor opposes it) when they find a way to get home.
• Vicki falls in love and leaves voluntarily.
• Steven leaves voluntarily in order to lead a peace effort.
• Katarina dies (sacrifices herself to save the Doctor).
• Sara Kingdom (dubiously a companion) dies by accident.
• Dodo leaves voluntarily (with no real reason given).

5.5 voluntary exits, 0.5 forced, and 2 deaths.

• Polly and Ben join the 1st Doctor by accident. They voluntarily leave when they have a chance to resume their normal lives.
• Jamie and Zoe are forced to leave by the Time Lords. (Jamie potentially came back in "Season 6b", and his second departure is unknown; given that the Doctor still ends up exiled to Earth, Jamie likely was forced to leave again).
• Victoria leaves voluntarily to live a normal life (much the same as Martha)

• Brigadier Lethbridge-Stewart played a similar role to a companion, but the Doctor left him, so doesn't really count for this question (and he died well after this). The same is true for Sergeant Benton and Mike Yates.
• Liz Shaw never travelled in the TARDIS, but was essentially a companion. She voluntarily left UNIT (with no real reason given).
• Jo leaves voluntarily (she falls in love).

• Sarah Jane Smith is forced to leave when the Doctor is summoned to Gallifrey. She is offered the chance to re-join the Doctor later, and declines.
• Harry leaves voluntarily to resume his normal life.
• Leela leaves voluntarily (she falls in love).
• K-9 (Mk I) leaves voluntarily (to stay with Leela) and K-9 (Mk II) is given to Romana (he can only survive in E-Space). K-9 (Mk III) and K-9 (Mk IV) didn't travel with the Doctor.
• Romana has a choice, but it doesn't include staying with the Doctor (she chooses between staying in E-Space and returning to Gallifrey, rather than either and staying with the Doctor).

3 voluntary exits, and 2 forced.

1 voluntary exit, 1 unknown.

2 voluntary exits, 2 forced.

• Amy and Rory leave unwillingly after a Weeping Angel sends Rory to 1930s New York, and Amy willingly chooses to be sent back along with him. The Doctor reading their tombstone creates a fixed point in time, so the Doctor is unable to rescue them.
• River is similar to a companion, although we don't really see much of her travelling with the Doctor. Her final appearance in The Husbands of River Song is after her official "death" in "Forest of the Dead".

• Like the Brigadier, Danny Pink never travels with the Doctor, but does play a similar role as a companion. He is killed when he is hit by an automobile, turned into a Cyberman, and eventually sacrifices himself to destroy the rest of the Cybermen, and sacrifices his return journey from the Nethersphere to send back a boy he killed during his time as a soldier.
• Clara Oswald is killed, then brought back between 2 heartbeats using Time Lord technology. She tricks the Doctor in forgetting about her, then leaves voluntarily for an adventure together with Ashildr in a stolen TARDIS. She will eventually have to return to her final moment so she can die.
• Bill leaves to explore the universe with Heather, voluntarily (her prior cyber-conversion doesn't seem to count as a death given the circumstances), although her only other choice was returning to Earth, not continuing to travel with the Doctor (much like Romana).
• Nardole is instructed by the Doctor to lead a group of people to safety, which he does. I consider this a forced exit.

1.5 deaths, 0.5 voluntary exits, 2 forced exits.