diff --git a/src/AcDream.Core/Physics/Motion/MotionState.cs b/src/AcDream.Core/Physics/Motion/MotionState.cs
new file mode 100644
index 00000000..94c96a58
--- /dev/null
+++ b/src/AcDream.Core/Physics/Motion/MotionState.cs
@@ -0,0 +1,122 @@
+using System.Collections.Generic;
+
+namespace AcDream.Core.Physics.Motion;
+
+/// <summary>
+/// One entry of a <see cref="MotionState"/> chain — retail's
+/// <c>MotionList</c> node (12 bytes: motion + speed_mod + next;
+/// acclient.h, r2-motiontable-decomp.md §14). One struct, two independent
+/// chain disciplines: the modifier chain is a PUSH-FRONT STACK, the action
+/// chain a TAIL-APPEND FIFO.
+/// </summary>
+public sealed class MotionEntry
+{
+    public uint Motion;
+    public float SpeedMod = 1f;
+
+    public MotionEntry(uint motion, float speedMod)
+    {
+        Motion = motion;
+        SpeedMod = speedMod;
+    }
+}
+
+/// <summary>
+/// R2-Q1 — verbatim port of retail's <c>MotionState</c> (acclient.h:31081;
+/// r2-motiontable-decomp.md §13): the motion-table-facing state block —
+/// current style / substate / substate_mod plus the modifier stack and the
+/// action FIFO that <c>CMotionTable::GetObjectSequence</c>,
+/// <c>StopSequenceMotion</c>, <c>re_modify</c>, and
+/// <c>MotionTableManager::AnimationDone</c> read and write.
+/// </summary>
+public sealed class MotionState
+{
+    /// <summary>Current style/stance command word (0 = unset; ctor 0x00525fd0).</summary>
+    public uint Style;
+
+    /// <summary>Current base substate command word (0 = unset).</summary>
+    public uint Substate;
+
+    /// <summary>Speed modifier of the base substate (default 1.0).</summary>
+    public float SubstateMod = 1f;
+
+    private readonly LinkedList<MotionEntry> _modifiers = new(); // modifier_head — push-front stack
+    private readonly LinkedList<MotionEntry> _actions = new();   // action_head/tail — FIFO
+
+    public MotionState()
+    {
+    }
+
+    /// <summary>
+    /// Deep copy (retail copy ctor 0x00526790; Q0-pins A4-#5): both chains
+    /// CLONED — <c>re_modify</c>'s snapshot is a termination bound, never
+    /// shared state.
+    /// </summary>
+    public MotionState(MotionState other)
+    {
+        Style = other.Style;
+        Substate = other.Substate;
+        SubstateMod = other.SubstateMod;
+        foreach (var m in other._modifiers)
+            _modifiers.AddLast(new MotionEntry(m.Motion, m.SpeedMod));
+        foreach (var a in other._actions)
+            _actions.AddLast(new MotionEntry(a.Motion, a.SpeedMod));
+    }
+
+    /// <summary>Modifier chain in retail order (newest first — push-front).</summary>
+    public IEnumerable<MotionEntry> Modifiers => _modifiers;
+
+    /// <summary>Action FIFO in retail order (oldest first).</summary>
+    public IEnumerable<MotionEntry> Actions => _actions;
+
+    /// <summary><c>add_modifier_no_check</c> (0x00525ff0): unconditional
+    /// push-front.</summary>
+    public void AddModifierNoCheck(uint motion, float speedMod)
+        => _modifiers.AddFirst(new MotionEntry(motion, speedMod));
+
+    /// <summary>
+    /// <c>add_modifier</c> (0x00526340): refuse when the motion is already
+    /// a modifier (caller must stop-then-re-add to refresh) or already IS
+    /// the current base substate.
+    /// </summary>
+    public bool AddModifier(uint motion, float speedMod)
+    {
+        for (var n = _modifiers.First; n is not null; n = n.Next)
+            if (n.Value.Motion == motion)
+                return false;
+
+        if (Substate == motion)
+            return false;
+
+        AddModifierNoCheck(motion, speedMod);
+        return true;
+    }
+
+    /// <summary><c>remove_modifier</c> (0x00526040) — by node identity
+    /// (retail's node+predecessor pair collapses in a managed list).</summary>
+    public void RemoveModifier(MotionEntry entry)
+    {
+        _modifiers.Remove(entry);
+    }
+
+    /// <summary><c>clear_modifiers</c> (0x00526070).</summary>
+    public void ClearModifiers() => _modifiers.Clear();
+
+    /// <summary><c>add_action</c> (0x005260a0): tail append.</summary>
+    public void AddAction(uint motion, float speedMod)
+        => _actions.AddLast(new MotionEntry(motion, speedMod));
+
+    /// <summary><c>remove_action_head</c> (0x00526120): pop the FIFO head,
+    /// returning its motion id (0 when empty).</summary>
+    public uint RemoveActionHead()
+    {
+        var head = _actions.First;
+        if (head is null)
+            return 0;
+        _actions.RemoveFirst();
+        return head.Value.Motion;
+    }
+
+    /// <summary><c>clear_actions</c> (0x005260f0).</summary>
+    public void ClearActions() => _actions.Clear();
+}
diff --git a/tests/AcDream.Core.Tests/Physics/Motion/MotionStateTests.cs b/tests/AcDream.Core.Tests/Physics/Motion/MotionStateTests.cs
new file mode 100644
index 00000000..41c51f98
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Physics/Motion/MotionStateTests.cs
@@ -0,0 +1,145 @@
+using System.Linq;
+using AcDream.Core.Physics.Motion;
+
+namespace AcDream.Core.Tests.Physics.Motion;
+
+/// <summary>
+/// R2-Q1 — verbatim <c>MotionState</c> (gap H2). Oracle:
+/// r2-motiontable-decomp.md §13 (ctor 0x00525fd0, add_modifier_no_check
+/// 0x00525ff0, add_modifier 0x00526340, remove_modifier 0x00526040,
+/// clear_modifiers 0x00526070, add_action 0x005260a0, clear_actions
+/// 0x005260f0, remove_action_head 0x00526120) + §14 (one node struct,
+/// two independent chains: modifier PUSH-FRONT STACK vs action TAIL-APPEND
+/// FIFO) + Q0-pins A4-#5 (deep-copy ctor clones both chains — re_modify's
+/// snapshot is a termination bound, never shared state).
+/// </summary>
+public class MotionStateTests
+{
+    [Fact]
+    public void Defaults_MatchRetailCtor()
+    {
+        var ms = new MotionState();
+        Assert.Equal(0u, ms.Style);
+        Assert.Equal(0u, ms.Substate);
+        Assert.Equal(1f, ms.SubstateMod);
+        Assert.Empty(ms.Modifiers);
+        Assert.Empty(ms.Actions);
+    }
+
+    [Fact]
+    public void Modifiers_ArePushFrontStack()
+    {
+        var ms = new MotionState();
+        ms.AddModifierNoCheck(0x0Du, 1.0f);
+        ms.AddModifierNoCheck(0x0Fu, 1.5f);
+
+        // Newest first — retail pushes onto modifier_head.
+        Assert.Equal(new uint[] { 0x0Fu, 0x0Du }, ms.Modifiers.Select(m => m.Motion).ToArray());
+    }
+
+    [Fact]
+    public void Actions_AreTailAppendFifo()
+    {
+        var ms = new MotionState();
+        ms.AddAction(0x62u, 1.0f);
+        ms.AddAction(0x63u, 1.25f);
+
+        Assert.Equal(new uint[] { 0x62u, 0x63u }, ms.Actions.Select(a => a.Motion).ToArray());
+    }
+
+    [Fact]
+    public void AddModifier_RejectsDuplicate()
+    {
+        var ms = new MotionState();
+        Assert.True(ms.AddModifier(0x0Du, 1.0f));
+        Assert.False(ms.AddModifier(0x0Du, 2.0f)); // already present → caller must stop-then-re-add
+        Assert.Single(ms.Modifiers);
+        Assert.Equal(1.0f, ms.Modifiers.First().SpeedMod); // original untouched
+    }
+
+    [Fact]
+    public void AddModifier_RefusesCurrentSubstate()
+    {
+        var ms = new MotionState { Substate = 0x45000005u };
+        Assert.False(ms.AddModifier(0x45000005u, 1.0f));
+        Assert.Empty(ms.Modifiers);
+    }
+
+    [Fact]
+    public void AddModifierNoCheck_SkipsBothGuards()
+    {
+        var ms = new MotionState { Substate = 0x45000005u };
+        ms.AddModifierNoCheck(0x45000005u, 1.0f);
+        ms.AddModifierNoCheck(0x45000005u, 2.0f); // duplicate allowed too
+        Assert.Equal(2, ms.Modifiers.Count());
+    }
+
+    [Fact]
+    public void RemoveModifier_ByNodeIdentity()
+    {
+        var ms = new MotionState();
+        ms.AddModifierNoCheck(0x0Du, 1.0f);
+        ms.AddModifierNoCheck(0x0Fu, 1.5f);
+        var target = ms.Modifiers.First(m => m.Motion == 0x0Du);
+
+        ms.RemoveModifier(target);
+
+        Assert.Single(ms.Modifiers);
+        Assert.Equal(0x0Fu, ms.Modifiers.First().Motion);
+    }
+
+    [Fact]
+    public void RemoveActionHead_PopsFifo_ReturnsMotion_ZeroWhenEmpty()
+    {
+        var ms = new MotionState();
+        ms.AddAction(0x62u, 1f);
+        ms.AddAction(0x63u, 1f);
+
+        Assert.Equal(0x62u, ms.RemoveActionHead());
+        Assert.Equal(0x63u, ms.RemoveActionHead());
+        Assert.Equal(0u, ms.RemoveActionHead()); // empty → 0 (retail returns 0)
+        Assert.Empty(ms.Actions);
+
+        // Tail cleared with the last pop — a fresh append works normally.
+        ms.AddAction(0x64u, 1f);
+        Assert.Equal(new uint[] { 0x64u }, ms.Actions.Select(a => a.Motion).ToArray());
+    }
+
+    [Fact]
+    public void ClearModifiers_And_ClearActions_AreIndependentChains()
+    {
+        var ms = new MotionState();
+        ms.AddModifierNoCheck(0x0Du, 1f);
+        ms.AddAction(0x62u, 1f);
+
+        ms.ClearModifiers();
+        Assert.Empty(ms.Modifiers);
+        Assert.Single(ms.Actions);
+
+        ms.ClearActions();
+        Assert.Empty(ms.Actions);
+    }
+
+    [Fact]
+    public void DeepCopy_ClonesChains_NoSharedState()
+    {
+        // A4-#5: re_modify's snapshot is a DEEP copy used as a termination
+        // bound — mutating the original must not touch the snapshot.
+        var ms = new MotionState { Style = 0x8000003Du, Substate = 0x44000007u, SubstateMod = 2.85f };
+        ms.AddModifierNoCheck(0x0Du, 1.5f);
+        ms.AddAction(0x62u, 1f);
+
+        var snap = new MotionState(ms);
+
+        ms.ClearModifiers();
+        ms.RemoveActionHead();
+        ms.Substate = 0x41000003u;
+
+        Assert.Equal(0x8000003Du, snap.Style);
+        Assert.Equal(0x44000007u, snap.Substate);
+        Assert.Equal(2.85f, snap.SubstateMod);
+        Assert.Single(snap.Modifiers);
+        Assert.Equal(0x0Du, snap.Modifiers.First().Motion);
+        Assert.Single(snap.Actions);
+    }
+}