diff --git a/src/libraries/System.Private.CoreLib/src/System/String.Comparison.cs b/src/libraries/System.Private.CoreLib/src/System/String.Comparison.cs
index 665e9b40feadf1..541045aa712aa1 100644
--- a/src/libraries/System.Private.CoreLib/src/System/String.Comparison.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Comparison.cs
@@ -523,7 +523,14 @@ public bool EndsWith(char value, StringComparison comparisonType)
         /// <returns><see langword="true"/> if <paramref name="value"/> matches the end of this instance; otherwise, <see langword="false"/>.</returns>
         public bool EndsWith(Rune value)
         {
-            return EndsWith(value, StringComparison.Ordinal);
+            if (value.IsBmp)
+            {
+                return EndsWith((char)value.Value);
+            }
+
+            UnicodeUtility.GetUtf16SurrogatesFromSupplementaryPlaneScalar((uint)value.Value, out char highSurrogate, out char lowSurrogate);
+
+            return Length > 1 && this[^2] == highSurrogate && this[^1] == lowSurrogate;
         }
 
         /// <summary>
@@ -1134,7 +1141,14 @@ public bool StartsWith(char value, StringComparison comparisonType)
         /// <returns><see langword="true"/> if value matches the beginning of this string; otherwise, <see langword="false"/>.</returns>
         public bool StartsWith(Rune value)
         {
-            return StartsWith(value, StringComparison.Ordinal);
+            if (value.IsBmp)
+            {
+                return StartsWith((char)value.Value);
+            }
+
+            UnicodeUtility.GetUtf16SurrogatesFromSupplementaryPlaneScalar((uint)value.Value, out char highSurrogate, out char lowSurrogate);
+
+            return Length > 1 && _firstChar == highSurrogate && this[1] == lowSurrogate;
         }
 
         /// <summary>
diff --git a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
index 343cf7bc9734d8..2d4ace7d1bfdda 100644
--- a/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs
@@ -1454,9 +1454,9 @@ private string ReplaceHelper(int oldValueLength, string newValue, ReadOnlySpan<i
         /// </returns>
         public string Replace(Rune oldRune, Rune newRune)
         {
-            if (Length == 0)
+            if (oldRune.IsBmp && newRune.IsBmp)
             {
-                return this;
+                return Replace((char)oldRune.Value, (char)newRune.Value);
             }
 
             ReadOnlySpan<char> oldChars = oldRune.AsSpan(stackalloc char[Rune.MaxUtf16CharsPerRune]);
@@ -1682,11 +1682,9 @@ public string[] Split(Rune separator, StringSplitOptions options = StringSplitOp
         /// <returns>An array whose elements contain the substrings from this instance that are delimited by <paramref name="separator"/>.</returns>
         public string[] Split(Rune separator, int count, StringSplitOptions options = StringSplitOptions.None)
         {
-            ReadOnlySpan<char> separatorSpan = separator.AsSpan(stackalloc char[Rune.MaxUtf16CharsPerRune]);
-
-            if (separatorSpan.Length == 1)
+            if (separator.IsBmp)
             {
-                return Split(separatorSpan[0], count, options);
+                return Split((char)separator.Value, count, options);
             }
 
             ArgumentOutOfRangeException.ThrowIfNegative(count);
@@ -1694,7 +1692,13 @@ public string[] Split(Rune separator, int count, StringSplitOptions options = St
             CheckStringSplitOptions(options);
 
             // Ensure matching the string separator overload.
-            return (count <= 1 || Length == 0) ? CreateSplitArrayOfThisAsSoleValue(options, count) : Split(separatorSpan, count, options);
+            if (count <= 1 || Length == 0)
+            {
+                return CreateSplitArrayOfThisAsSoleValue(options, count);
+            }
+
+            ReadOnlySpan<char> separatorSpan = separator.AsSpan(stackalloc char[Rune.MaxUtf16CharsPerRune]);
+            return Split(separatorSpan, count, options);
         }
 
         // Creates an array of strings by splitting this string at each
@@ -2411,39 +2415,28 @@ public unsafe string Trim(char trimChar)
         /// </returns>
         public string Trim(Rune trimRune)
         {
-            if (Length == 0)
+            if (trimRune.IsBmp)
             {
-                return this;
+                return Trim((char)trimRune.Value);
             }
 
-            // Convert trimRune to span
-            ReadOnlySpan<char> trimChars = trimRune.AsSpan(stackalloc char[Rune.MaxUtf16CharsPerRune]);
+            UnicodeUtility.GetUtf16SurrogatesFromSupplementaryPlaneScalar((uint)trimRune.Value, out char highSurrogate, out char lowSurrogate);
 
             // Trim start
             int index = 0;
-            while (index < Length && this.AsSpan(index).StartsWith(trimChars))
+            while ((uint)(index + 1) < (uint)Length && this[index] == highSurrogate && this[index + 1] == lowSurrogate)
             {
-                index += trimChars.Length;
-            }
-
-            if (index >= Length)
-            {
-                return Empty;
+                index += 2;
             }
 
             // Trim end
-            int endIndex = Length - 1;
-            while (endIndex >= index && this.AsSpan(index..(endIndex + 1)).EndsWith(trimChars))
-            {
-                endIndex -= trimChars.Length;
-            }
-
-            if (endIndex < index)
+            int endIndex = Length - 2;
+            while (endIndex > index && this[endIndex] == highSurrogate && this[endIndex + 1] == lowSurrogate)
             {
-                return Empty;
+                endIndex -= 2;
             }
 
-            return this[index..(endIndex + 1)];
+            return this[index..(endIndex + 2)];
         }
 
         // Removes a set of characters from the beginning and end of this string.
@@ -2497,24 +2490,17 @@ public unsafe string Trim(params ReadOnlySpan<char> trimChars)
         /// </returns>
         public string TrimStart(Rune trimRune)
         {
-            if (Length == 0)
+            if (trimRune.IsBmp)
             {
-                return this;
+                return TrimStart((char)trimRune.Value);
             }
 
-            // Convert trimRune to span
-            ReadOnlySpan<char> trimChars = trimRune.AsSpan(stackalloc char[Rune.MaxUtf16CharsPerRune]);
+            UnicodeUtility.GetUtf16SurrogatesFromSupplementaryPlaneScalar((uint)trimRune.Value, out char highSurrogate, out char lowSurrogate);
 
-            // Trim start
             int index = 0;
-            while (index < Length && this.AsSpan(index).StartsWith(trimChars))
+            while ((uint)(index + 1) < (uint)Length && this[index] == highSurrogate && this[index + 1] == lowSurrogate)
             {
-                index += trimChars.Length;
-            }
-
-            if (index >= Length)
-            {
-                return Empty;
+                index += 2;
             }
 
             return this[index..];
@@ -2571,27 +2557,20 @@ public unsafe string TrimStart(params ReadOnlySpan<char> trimChars)
         /// </returns>
         public string TrimEnd(Rune trimRune)
         {
-            if (Length == 0)
+            if (trimRune.IsBmp)
             {
-                return this;
+                return TrimEnd((char)trimRune.Value);
             }
 
-            // Convert trimRune to span
-            ReadOnlySpan<char> trimChars = trimRune.AsSpan(stackalloc char[Rune.MaxUtf16CharsPerRune]);
+            UnicodeUtility.GetUtf16SurrogatesFromSupplementaryPlaneScalar((uint)trimRune.Value, out char highSurrogate, out char lowSurrogate);
 
-            // Trim end
-            int endIndex = Length - 1;
-            while (endIndex >= 0 && this.AsSpan(..(endIndex + 1)).EndsWith(trimChars))
+            int endIndex = Length - 2;
+            while ((uint)endIndex < (uint)Length && this[endIndex] == highSurrogate && this[endIndex + 1] == lowSurrogate)
             {
-                endIndex -= trimChars.Length;
-            }
-
-            if (endIndex < 0)
-            {
-                return Empty;
+                endIndex -= 2;
             }
 
-            return this[..(endIndex + 1)];
+            return this[..(endIndex + 2)];
         }
 
         // Removes a set of characters from the end of this string.
diff --git a/src/libraries/System.Private.CoreLib/src/System/String.Searching.cs b/src/libraries/System.Private.CoreLib/src/System/String.Searching.cs
index f2218eb276603b..3120477f686e01 100644
--- a/src/libraries/System.Private.CoreLib/src/System/String.Searching.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.Searching.cs
@@ -52,6 +52,11 @@ public bool Contains(char value, StringComparison comparisonType)
         /// <returns><see langword="true"/> if <paramref name="value"/> occurs within this string; otherwise, <see langword="false"/>.</returns>
         public bool Contains(Rune value)
         {
+            if (value.IsBmp)
+            {
+                return Contains((char)value.Value);
+            }
+
             return Contains(value, StringComparison.Ordinal);
         }