I see pretty much every weapon in DMC like the soul of a demon. But as Dante, Vergil and Sparda, every higher demon has a human form. The default form varies depending on if they are born demons (demon form = default) or if they are half-demons like Dante and Vergil (human form = default).
I'm from the school of DMC thought where I think every demons devil trigger is based on their demonic soul, and that defines their abilities and form. I also think that because Dante and Vergil (and sure.. bring Nero along too) is half demons, their demonic form cannot be accessed without the help of another demon's soul.
That is why in DMC1 and DMC3, that Dante changes his devil-trigger form depending on the weapon his using. Half of the devil-trigger form is his own, the other half is the weapons demon form. So Dante doesn't transform into the Alastor demon during devil trigger, but is rather a hybrid between the two. So I think the Alastor demon probably looks very different from Dante. And side note: because Dante and Vergils own half-demon form is added, they would probably look very different from each other when taking the form of the Alastor demon. And remember, even though Dante and Vergil are twins, they would look different when taking the devil trigger form of the same weapon because its the souls that fuses, not their physical appearance as much.
Most of the weapons are devil arms, but I see some of them as just magical, demonic relics or whatever without a demonic form. If Dante/Vergil could activate devil trigger while using these relics, it would probably mainly be their half demon form that was displayed. Maybe with a few changes based on the magical properties of the weapon.
SO YEAH!!! Lets just do DMC1 right now.
You'd think they had some forms in between too.. Just because Japan likes different forms.
FORCE EDGE - Just a relic
No demonic or human form.
ALASTOR - DevilArm
Devil form = blue glowing eyes without pupils, dark purple skin with blue glowing between the muscle masses, armor stylized with a dragon like the blade, slightly fured like a werewolf on chest, back, shoudlers and legs. Large demon wings with lightning flowing through the bloodvains. Semi-short/long horns bending backward for aerodynamics (lol). Thick long tail in typical demon fashion. Four arms... or whatever.
Human form = thin atletic body. Semi long gray hair. Black textile bandage around the head covering the eyes. Eyes glow blue under the bandages when transforming. Royal simplistic robe clothing in black with blue details. Bandages around arms and hands. Sharp nails.
IFRIT - DevilArm
Devil form = Think LotR movie Balrog / bull demon...
Human form = Large bearded man. Think old concept art Spardas human where is is frikkin' huge and with beard.
SPARDA - DevilArm
You all know spardas human and devil form.
YAMATO - Just a relic
No demonic or human form.
Meeh....