because your companions choose what they wear for themselvs. The "Armor Upgrades" are items you find through the story that are character specific. I've made a list of the upgrades and when you can obtain them.
Carver:
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Act 1: misson - birthright name of the armor - Fereldan girded plating.
Bethany:
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Act 1: Misson - birthright. Name is Heirloom amell protective sigils.
Aveline:
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Act 1: Underpadding - Guardsman pattern. You'll find it on the armor stand in lowtown.
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Act 2: Impact Plating - Guardsman pattern. Yet again on the armor stand in lowtown.
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Act 2: Flex-Chain - Guardsman pattern. Mission - Raiders on the cliffs
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Act 3: Deflecting Joints - Guardsman pattern. Misson - Favor and Fault
Varric:
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Act 1: Inscribed leather Harness, Found in the apparel shop in lowtown.
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Act 2: Coat Lining with Concealed Pockets, Found in the shady merchandise shop in the docks.
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Act 2: Silverite-Reinforced Buckles. Misson - Family Matter
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Act 3: Drakeskin leg Straps. Misson - Finding Nathaniel. Note: this requires a import of Dragon age: Awakenings where Nathaniel was not killed.
Anders:
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Act 2: Armor struts. Found at Lirene's ferelden Imports, in Lowtown,
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Act 2: Lyrium weave. Found at mage goods in the Gallows courtyard.
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Act 2: Spirit Essence. Misson - Dissent
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Act 3: Sigil of the mage underground. Misson - Best served cold
Merril:
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Act 2: Samite Liming, Found at Robes by kean luc in hightown.
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Act 2: Carved Ironwood Buttons, Found at Ilen's Crafts in sundermount.
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Act 2: Silver-Threaded Dalish Embroidery, Found at the top of sundermount.
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Act 3: Halla horn Buckles - Misson - A New Path
Isabela:
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Act 2: Rigid Boning. Found in the appearl shop in lowtown.
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Act 2: Supportive Corselet. Found in Robes by jean Luc in hightown
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Act 2: Lambswool Insoles. Misson - To Catch a Thief
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Act 3: Boiled Leather Plates. Misson - A murder of Crows Note: this requires a import where zevran did not die.
Fenris:
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Act 2: Tevinter Spirit symbol. Found at robes by jean Luc in hightown.
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Act 2: Lyrium Scales. Found at the shady merchandise shop in the docks.
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Act 2: Reinforced straps. Misson - Bitter pill
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Act 3: Enchanted Resin. Misson - Mine Massacre
Sebastian:
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Act 2: Enhanced Articulation. Found at the Armor shop in the gallows courtyard.
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Act 2: Reinforced Bracers. Found at Olaf's Armory in hightown
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Act 2: Mail Undertunic. Mission - Repentance
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Act 3: Protection of the Faith. Misson - Best served Cold.