Jethro (formerly known as 'Butch') is a drug-sniffing dog, first owned by deceased Navy Petty Officer Kyle Hanson and later by NCIS Special Agent Timothy McGee.
Biography
Pre-Series
Presumably born on a Naval base or as a result of a breeding program, Butch or Jethro was raised by various people, eventually learning how to detect drugs.
He was also presumably owned by numerous people before falling under the command of Navy Petty Officer Kyle Hanson.
NCIS Season 5
In Dog Tags, the team attempt to arrest a Navy Petty Officer who is suspected of dealing drugs. Whilst doing so, McGee is attacked by Butch - then assumed to be the Officer's dog. McGee is forced to open fire on Butch, injuring him but not seriously, as Donald Mallard remarks. Butch is taken back to NCIS as evidence, owing to his relevance in the case.
Abigail Sciuto is horrified that McGee shot Butch, and takes care of Butch whilst he counts as evidence, growing attached to the dog. She renames Butch as 'Jethro', citing that 'Butch' was too butch. However, despite her attempts to refute the idea, evidence increasingly points to Jethro being the Officer's killer. It's decided that Jethro must be put down. Erica Perelli - a dog handler from Pax River - visits NCIS to collect the dog, but both he and Sciuto have disappeared to 'gather evidence'. At Pax River Naval Base, Jethro digs up the skeleton of an unnamed dog.
Back at NCIS, McGee makes another attempt to collect Jethro, but Sciuto has locked herself and the dog in the lower part of her lab, blasting The Cure's 'The Lovecats' to drown McGee out. Sciuto refuses to open the door until Jethro is proven innocent, despite McGee's pleas that she doesn't cause a scene. Later, Director Shepard visits Sciuto, who immediately opens the door for her boss. She wonders if Sheppard will order her to give up Jethro, but she doesn't, instead speaking to her about how sometimes things are out of your control. Sciuto however refuses to relent in her quest to prove the Dog's innocence.
Jethro later begins to cough up blood, leading Sciuto to take him quickly to Mallard and James Palmer, who detect an anomalous item in his stomach with an X-ray. In a hurry they surgically remove the item from Jethro, determining it to be the tip of a knife. This cleared up confusion that Mallard had about an odd puncture mark in the officer's collarbone, determining that the tip of that knife fitted perfectly. This led him to conclude that it was a knife that killed the officer, not the dog bites Jethro left. The tip also fits perfectly the wound of the unnamed dog, showing the two killings were related.
The team learn that the knife tip came from a Navy issue Tactical Boot Knife, only one having been ordered within the last 72 hours - by Perelli. It transpires that she had killed the officer, and tried to make it look as though Jethro had killed him in a violent dog attack. This absolved Jethro of murder.
Sciuto is later found by McGee holding onto Jethro, sad that she can't keep him because her landlord doesn't allow pets. She suggests that McGee take him into his ownership, stating that Jethro deserves to be happy, and that McGee was already looking for a dog. McGee isn't happy about it but Sciuto insists, bringing them together and ordering them to shake hands.
From then on it's understood that Jethro lives with McGee as his pet. Jethro gets few appearances/mentions after Dog Tags, however McGee mentions him a few times, referencing how he's grown greatly attached to him. Anthony DiNozzo jokes about how Jethro may as well be his girlfriend, to which McGee reminds him that Jethro is a boy.
In later years, with McGee having gotten married to Delilah Fielding and the couple having twins, it's presumed that McGee either gave up Jethro for adoption elsewhere or that Jethro might have died of old age.
Personality
At first Jethro (then 'Butch') is seen to be incredibly aggressive, attacking McGee and injuring him badly - making him wary of Jethro. However quite quickly it's seen that Jethro is a playful dog at heart, as seen when Sciuto dances with him to Nellie McKay's 'The Dog Song'. It's established that Jethro was spooked and panicked when the team came across him, and that actually he is relatively peaceful. McGee eventually realises this and becomes attached to Jethro as his owner.
Physical Appearance
Jethro is a relatively large German Shepherd with brown.