Fenno was a hefty male shrew of the Guosim tribe. He bullied Dippler because it was the younger shrew's fault that all the logboats were stolen by Marlfoxes. The Log-a-Log was protective of Dippler, though, and insulted, threatened and hit Fenno whenever he abused the young shrew; as a result, Fenno became very resentful of Log-a-Log's authority. Things came to a head when Fenno voiced his disinclination to defend Redwall Abbey from the Marlfoxes, not wanting the Guosim to be "killed for creatures that ain't even shrews!" Log-a-Log planned to duel it out with Fenno after the battle with the Marlfoxes ended; this, however, never came to pass.
During the Guosim's charge against the Marlfoxes, Fenno tripped and fell. Log-a-Log unwiseley ran past his downed foe, and Fenno seized the opportunity to throw his rapier and slay his chieftain. He then ran away, but was captured by Mokkan and his followers; the Marlfox forced Fenno to steer their stolen logboat down the River Moss towards Marlfox Island. True to his nature, Fenno tried to get the fox's water rat soldiers to turn against him; Mokkan overheard, and badly beat the shrew to teach him a lesson.
Fenno finally managed to sneak off the logboat when Mokkan and his water rats were attacked by lizards in a watermeadow. Later, he met Dippler again and challenged the young shrew to a duel; in the ensuing swordfight, Fenno came off for the worse and was killed.
Not to be confused with Abbess Fenna and Fenn Bluepaw.