Captain Sainfoin was captain of the Neck Mark.
In the Miniseries, Sainfoin is a sergeant instead of a Captain and is the tertiary antagonist. He is portrayed as a heavily built rabbit, almost as large as Woundwort, with dark grey fur and brown eyes. He is a rough and somewhat rude bully who often strikes and threatens other rabbits for no good reason, but like most bullies he is a coward at heart.
Sainfoin first appears when he informs Woundwort that one of the Wide Patrols picked up the trail of a large group of rabbits (the Watership Down rabbits), but lost the scent. Woundwort orders him to tell the patrol not to return until they are located and to leave one alive for questioning.
When Holly, Blackberry and Bluebell are taken to Efrafa by Captain Orchis, He is seen guarding Hyzenthlay, Thethuthinnang and Nettle, While they are being reprimanded by Vervain for trying to escpae Efrafa, Vervain himself turns to Sainfoin and asks if he is in charge of them, Sainfoin is slow to respond and his pinned to the ground by Vervain who than orders him to get them down to the hole.
Not long after, Vervain orders him to take Blackavar down to the deep burrows, After the Council decide to have the latter marched around each of the marks at silflay as an example, Sainfoin gladly obeys.
Later, while on wide patrol with another soldier, Sainfoin comes across Clover wandering around in a fog and forcibly takes her to Efrafa, where she is inducted into the ranks of does.
When Bigwig is taken to Efrafa, Sainfoin comes to inspect him instead of Woundwort. Bigwig, who expected to meet the Chief Rabbit, is told that the General is not so easily met. Presenting himself under his Lapine name of Thlayli, Bigwig claims to be a storyteller whose warren was destroyed by men and has traveled to Efrafa with the intention to join the warren. Sainfoin, noticing Bigwig’s size, suspects Bigwig of having originally been a fighter and strikes him across the face. When Bigwig doesn’t fight back, Sainfoin is convinced of Bigwig’s story. A guard then approaches the two rabbits and informs them of a fox's attack on Campion’s Wide Patrol led onto them by an unknown rabbit, not knowing that the rabbit was Bigwig. Due to the loss of guards in the recent days, Sainfoin decides to induct Bigwig into the ranks of the Owsla to be trained as a fighter.
Later, when Captain Orchis questions Bigwig, suspecting him of being a spy, Sainfoin strikes him across the face again at Orchis's signal. Bigwig still doesn't fight back, however, not wanting to blow his cover. He is later present during Hyzenthlay's execution and is ordered to kill Bigwig, the appointed executioner, if he does not complete the task. Bigwig ultimately refuses to kill Hyzenthlay, causing Sainfoin to strike him across the face yet again. Bigwig finally fights back and easily overpowers Sainfoin, venting his anger and frustration on the Efrafan. Sainfoin retreats.
Sainfoin is part of the Efrafan army when they surround the escaped rabbits on all sides. However, when Kehaar appears and scatters the Efrafans, Sainfoin quickly retreats with them, but not before being briefly knocked over by the gull.
Early the next morning, Sainfoin meets with Campion and Orchis in the Council Chamber to privately discuss the recent events. For the first time, Sainfoin shows a degree of sympathy and common sense, questioning why the Efrafans should be chasing after a group of rabbits who never did them any harm in the first place. Campion agrees with the sentiment. Sainfoin also begins to question Woundwort, pointing out everything they do is on his orders. Tensions rise between the three officers before Woundwort appears in the burrow and requests that all the other guards in Efrafa convene in the burrow. Sainfoin goes off to obey the command.
During the subsequent meeting, Woundwort reveals that he invited the Captains into the burrow to ask them to voice any concerns that they might have. After a moment of silence, Sainfoin expresses his reluctance to go after the Watership Down rabbits, claiming that he fees the lives of the soldiers will be risked as well as wondering what other creatures the outsiders have under their control, pointing out how the rabbits have a bird under their control. Disgusted by Sainfoin’s cowardice and fear, Woundwort berates Sainfoin before forcefully asking for his loyalty, to which Sainfoin complies.
During the siege of Watership Down, Sainfoin repeatedly expresses fear that Kehaar might still be around, but falls silent when Woundwort threatens to kill him. He is chosen to go into the enemy warren with General Woundwort after Campion refuses to do so, though it is more likely he obeys out of fear than loyalty. He watches Woundwort and Bigwig's fight without attempting to help and is greatly shocked when Bigwig reveals he is not the Chief Rabbit. Sainfoin goes back to the surface and reveals the news to the remaining troops. A badly injured Woundwort then emerges from the warren and, acting as if nothing were wrong, insists that the war is nearly over. However, the news about the Chief Rabbit and shock at Woundwort's increasing madness finally causes Sainfoin to desert the General to save his own life. He is again denounced as a coward by Woundwort as he flees the down with several other Efrafans. It is unknown what happened to him after that.