Isabelle Sinclair, or simply Izzy, is a fictional character who appeared in the Doctor Who Magazine comic strip based on the long-running British science fiction television series Doctor Who. She was a companion of the Eighth Doctor. The canonicity of the comic strips, like other Doctor Who spin-off media, is unclear.

Izzy was born on October 12, 1979 (the same day that the first issue of Doctor Who Weekly was published), but she did not know where, nor did she know who her biological parents were. She was the adopted child of Lee and Sandra Sinclair, and a young science fiction fan and paranormal investigator who lived in the town of Stockbridge, England. She was a friend of Maxwell Edison, a UFO enthusiast who had encountered the Fifth Doctor several years before.

She was introduced in End Game, which ran in issues #244-#247 of DWM, written by Alan Barnes and drawn by Martin Geraghty. When she first met the Doctor, she and Max had stolen an artifact that was being sought by the Doctor's old enemy the Celestial Toymaker . The Toymaker had taken the entire town of Stockbridge into a pocket dimension and turned everyone into dolls save for Izzy and Max. The Doctor arrived to rescue his friends and defeat the Toymaker once again, and took Izzy on board the TARDIS as his newest companion

Izzy had left Earth without telling her adoptive parents, but that did not concern her much as she was trying to escape what she felt was a too-mundane life. As a result, she revelled in the experience of travelling with the Doctor, even though it was usually dangerous. However, her vivacious and trusting nature eventually had tragic consequences. When the TARDIS was swallowed by a gigantic snake-like spacecraft called Ophidius, Izzy and the Doctor met Destrii, an amphibious alien with the characteristics of a humanoid fish. Destrii was being pursued by the group of aliens that controlled Ophidius, and gained Izzy's confidence only to switch bodies with her to evade capture. When Destrii (in Izzy's body) was apparently disintegrated, Izzy had to deal with the possibility that she would be in an alien body for the rest of her life.

Just as she began to adjust to her new body, Izzy was kidnapped by operatives of Destrii's mother, the Matriax Scalamanthia of the planet Oblivion, who wanted her "daughter" to prepare for her wedding by fighting a duel to the death. While trying to trace Izzy's whereabouts with the help of his former companion Fey, the Doctor discovered that Destrii was still alive, and took her to Oblivion to switch their bodies back. In the process, both Destrii and Izzy saw into each other's minds, and Destrii saw how finding out she had been adopted made Izzy withdraw, unable to trust herself or admit that she was a lesbian. Finally coming to terms with who she was, Izzy asked the Doctor to take her back to Stockbridge so she could make amends to her adoptive parents. The Doctor dropped Izzy off at the point where she began travelling with him, so that from her parents' point of view, she had never left.

Izzy was the Eighth Doctor's most constant companion in the comic strip, appearing in nearly all of the Eighth Doctor's appearances between 1996 and 2003. Her time with the Doctor saw the coming and going of both Kroton and Fey as companions. An older Izzy made a cameo appearance in the last regular Eighth Doctor comic strip story, The Flood (DWM #353). She also appears in the short story Syntax by David Bailey, published in the Big Finish Productions anthology, Short Trips: Life Science, set during her time with the Doctor.

• Travelled from: End Game (DWM #244-#247) to Oblivion (DWM #323-#328)