Although he has his fighting moments—he kills Baligant's son Malprimes and has an excru-ciatingly close call with Baligant's brother, Canabeus—he's primarily a man of advice. Unfortunately, the most important advice he gives is bad. Yep, he was the main guy promoting Ganelon's idea that Marsile was telling the truth, convinced that it would be unjust to attack a man who professes Christianity and service.
Unlike Roland, who relies on intuition and an abstract sense of honor, Naimes takes things at face-value and always advises based on the evidence of his senses, like when he hears Roland blows the oliphant (135) or sees the dust rising from Marsile's fleeing army (178). This can be good, like when he realizes that oliphant = Roland in trouble.
But face-value can also be deceptive. Although Charlemagne admits that he can't know Marsile's mind from his words (13.191), Naimes is immediately convinced that Marsile means what he says. It's the hostages that win him over:
"Since he wishes to reassure you by hostages,
This great war must no longer continue." (16.241-42)
Naimes himself is far too straightforward to suspect others of deceit. Anyone who sends his own men to be killed by the enemy must be sincere.
Because he's such a well-respected and valuable knight, Charlemagne listens closely to what he says and refuses to let him be far away. When Naimes volunteers to be the envoy to Marsile, Charlemagne snaps like a turtle:
"You're a valuable man;
By this beard and by this moustache of mine,
You'll never go so far from me.
Go sit down, no one has called upon you!" (17.248-51)
God also treasures him, since he goes out of his way to protect him from what should have been a deathblow from Canabeus:
He would have fallen had God not helped him. (248.3439)
A lot of Frankish knights die under the swords of the Emir's army, but only Naimes and Charlemagne get divine anti-death intervention.
Sometimes wise but definitely not intuitive, Duke Naimes demonstrates how Charlemagne's practice of taking advice from his council can be both valuable and terrible.